Any specific recommendations?

I've played "all" the fan missions made up to 2007 for Thief 1 and one stands out from the rest as being on par with Lookingglass' own missions: Calendra's Cistern.
There are other good ones, but I can't recall their names now.

Thanks!

I downloaded Calendra's Cistern, but I got the impression that it is supposed to be a series of missions, yet the archiver says that it is missing something (presumably other missions?).

I played through Shadows of the Metal Age a week ago.

It's 5 years since I played it, but I can't remember any problems installing it.
It was only one large mission to my knowledge, but I seem to recall there being talk about sequel(s) for Thief 2, but I don't know of they materialized.
I don't follow the Thief fan missions community anymore (don't have time for it) so I can't be more specific.

Calendra's Cistern is a single, large, complete mission and it's definitely on of the top fan missions for Thief 1 / Gold. While it wasn't the first fan mission for Thief 1 it was a breakthrough on many levels and already featured Purah's great urban architecture with many mysteries and tiny nooks to find stuff in.

However, there is a sequel called "Calendra's Legacy" for Thief 2 which is a mini-campaign, consisting of three large missions. It continues the story of Calendra's Cistern but one can play it independently (I recommend to play Cistern first, though).
This is considered an all-time classic by many fan mission players and it was probably the first high-quality campaign for Thief 2. While it's a team effort it shows even better level construction and is way above the regular Thief games, graphics-wise. Expect to invest a day into each of the three missions.
It was the last Thief mission by Purah (Anthony Huso) who went on to work for Arkane Studios after that (worked on Dark Messiah of Might & Magic and the new Dishonored).

If you like city missions like the Calendra series then be sure to check out the missions by Melan (Gabor Lux) - available for download here:
http://thiefmissions.com/search.cgi?search=melan&sort=title#m

No Thief game has a mod manager built into it (not counting The Dark Mod) so you have to use external applications: "DarkLoader" for Thief 1+2 and "GarrettLoader" for Thief 3 (don't use it for Thief 1+2, even if it says that it's supporting them).
There is a new loader in the works that caters for all Thief games but AFAIK it's not really finished/stable. Everybody seems to use DarkLoader for the first two games as the most recent version (v4.3) it is by far the most stable software for this.
Essentially, you put your downloaded missions in a directory somewhere, point DarkLoader to it (in the settings dialogue) and it then displays a list of missions from which you can select and start one.
The only problem I have with Thief 1 is that it doesn't support the mouse wheel natively (in my case to switch the objects in my inventory) so that I have to use a tool (AutoHotkey) with a batchfile to map the mousewheel to the desired keys.
When I use DarkLoader to start a mission it calls the Thief EXE directly but not the AutoHotkey batchfile (the EXEs are "hardwired" so to speak) so I use DarkLoader only to switch fan missions or back to the original game and then start Thief 1 via a desktop link to the batchfile which in turn starts AutoHotkey and the Thief 1...
The reason I do this is to get practically the same key and mouse config for both Thief 1+2.

As for patches/fixes/mods you are fine with TFix for Thief 1!
It really does contain almost anything you may need for compatibility and content enhancement, except the AutoHotkey stuff I mentioned above for utilizing the mouse wheel. I also loved Dark Messiah, mostly for the "feel" of it (fights, physics, atmosphere) but I assure you that the level construction, while beautiful, is a far cry from Purah's best work on Thief...

Purah has a great urban style and is definitely an artist and what we call a "master builder" in the Thief forums. Sadly, his professional progress left the Thief world without new missions from him. There are quite a lot of fantastic stuff from other mission authors, though, so you'll have months of gameplay ahead of you (even though most of it is for Thief 2).
As I've written befor check out Melan - his bigger missions are at least on the same level as Purah's best work (Calendra's Legacy). While perhaps not as playful they are often grittier and even more intricate, while keeping a more realistic look. Highly recommended!

The great thing about the Thief series is that you can really play them like *you* want.

Play them on easy the first time. Then discover that the higher difficulty settings bring more mission objectives, more enemies and often open up additional areas. If you are like me then you get angry with yourself because you missed out on so much and play the game *directly* a second time after this.

Then you discover the countless fan mission and get better playing it all the time until you don't need to fight or flee anymore. You only fight if you want to (and you discover how to kill the undead relatively easy) and flee if you employ it as a tactic.
You then discover some of the glitches in the game that the developers missed out like dropping from big heights on the heads of NPCs to survice the fall or the bunny jumping in Thief 1...

Then you learn to "ghost" a mission which means that you don't get seen from anybody. At this point you are really becoming a master thief and not Joe Random Burglar or the knockout king of Newport.
This can be incredibly time consuming, learning guard patterns etc., but also very satisfying. You also get better playing this way and the game becomes even easier.

Is ghosting getting too easy for you? Then play it the "Lytha Way": No alarms, not seen by anyone (including animals), obviously no killings (not even spiders), no KOs, don't break anything, no doors left open etc.
Still not hard enough? Don't save! ;-)

Personally, I always play on expert (unless there is good reason against it) but I love to knockout some of the more obnoxious NPCs so I play my own style, depending on how much time I have and how good the mission is. Some missions I race through, others I really comb to find the maximum amount of loot and secrets.

The Thief games have really become a big playground with it's own lore and they are among the oldest single-player PC games being played today for a good reason: Freedom.
How many ten year old games do you know that still get new fan missions?
Sadly, I'm not very optimistic that Thief 4 will support mods/missions at all so get on the classic Thief boat while you still can! ;-)

Yup, that's what I love about the Thief games - open, unlinear levels and you (or Garret) are the hunter, not the hunted. The oldest one is probably Forgotten Realms Unlimited Adventures from 1993. Not a game in itself, though, but an editor based on the classic Gold Box game. The community is still alive, and new modules are still being made, although there was a significant drop in activity when Neverwinter Nights was released in 2002.
Otherwise Doom is probably the oldest one if new WADs are still made for it (I don't know).

I loved the first two Thief games. I finished them both a few weeks ago, and greatly regretted not actually trying harder when I was younger.

I honestly never tried them on "Normal" difficulty. I saw all the extra objectives on Expert and felt like I was missing out on a huge chunk of the game if I didn't play it on that. The text for the objectives comes off as an internal monologuelike checklist, and it kind of felt like Garrett was not the type of person to kill the people he thieves on as a matter of professional pride.

Some year I'll have to try ghosting it.

I've really enjoyed the freedom as you mention it, and am kind of sad that there were only so many Looking Glass scenarios.

I'm planning on playing Thief 3 once these exams finally let me off the hook, so we'll see how that goes. I'll also have to find the time to figure out the Thief 1 missions.

For Thief II check out T2X: Shadows of the Metal Age. It has a multi-mission storyline, voice acting, etc.

Thanks!

I finished that one the other week as well.

I really apologize for not clarifying what I have completed so far. I've finished Thief 1 and 2, as well as T2X.

It was great, though I felt like the knockout hammer was potentially gruesome and kept thinking that I was going to crack some skulls. The timeline fit fairly perfectly in line with The Metal Age too. I read on some reviews that the levels were extremely confusing and poorly designed, though I was not sure where they were coming from.

For the life of me I could not get DarkLoader to work with Thief 1, so I just did the "Without Darkloader" installation for Calendra's Cistern.

It's big. I will definitely say that it is confusing as well. I am at 3500ish loot and have not been able to figure out how to crack the safe.

I'm loving just exploring things, even though the design is a tad claustrophic.

Edit: Started Calendra's Legacy.... apparently you're not allowed to pickpocket...?
Was not expecting the "wait several minutes" part.

That's strange! Which version of DarkLoader? 4.3?
Did you optimize for Thief 1? It should ask you right after you set it up.
Even if you managed to get Cistern up running you should clear this up as there are other really nice missions for Thief 1. Give it time - it's all in there. And more! You will find body parts of a deceased priest in various parts of the mission, for example. See what you can do with them! It reminds me of middle age Europe where the towns were crammed with small houses with low ceilings. For me this design is actually refreshing as I don't like to run around for minutes without something happening. I'd rather progress more slowly and carefully inspect my surroundings. Believe me, there is way more to the Calendra missions than meets the eye at first!

Admittedly, Cistern is a heavy-weight!. It's way more complicated than most original Thief missions and it's not as linear as most ("linear" as in "one always *knows* what to do next").
Tackling the Calendra missions and wanting to explore every nook and cranny in them after just completing the original missions is like advancing from cart racing to formula 1, IMHO. ;-)

It's well worth getting through them, though, as you will be able to complete every mission.

Here are a few mission suggestions for Thief 1 that may prove equally complex or easier:
- "Sepulchre of the Sinistral" - probably the best dungeon only mission for Thief 1, complex & lots of undead!
- "The Shadow of Lord Rothchest" - first mission from Purah, less complex but already a promising design (hint: ALWAYS search for underground or burial sites in his missions!)
- "Autumn in Lampfire Hills" - also from Purah (the title being a play on the old Sinatra song. obviously)
- "Lorgan's Web" - a spin on a original city missions
- "The Secret Way" - another very nice city mission
- "Bloodstone Prison" - one of the best "escape from a dungeon cell scenarios"
- "Trial by Night" - you are a thief apprentice and up for guild trial
- "Shadow Politics" - a recent goodie for Thief Gold

And these are only for Thief 1/Gold - there are way more good ones for Thief 2! I can't remember not being allowed to pickpocket or to actually need it in the first mission but you have to be discreet as the mission objectives say. So no FAILED attempts at thievery!
Hint: You can also inspect & plunder the guard-house by sneaking around without getting noticed.

However, the first mission does indeed have a special feature: You can either directly meet up with Basso and conclude the mission in under two minutes (see also the loot requirements...) or you can explore it and spend hours(!) in it - it's filled with the stuff of legends (hint hint... ;-)). Explore everything thoroughly and remember that the mission author is skilled at making up legends and writes about them. Buried legends that could prove true...

Look, this three mission campaign has gotten a huge amounts of love amongst taffers and not without reason.
There are several threads on TTLG for it - whether for content & solutions, technical problems or discovering secrets - and you should probably dig there. While I played the campaign three times (and yes, I missed out on some stuff the first time, too) it's been a while and as much as I want to help you out I can only vaguely point you into some directions.

I found just about everything in the first level, but have found out that there is no way to get into the jewelry shop in the second level after the zombie hordes come out.
I just spent a few hours fighting a guerilla war against the zombies to try and make it easier to move around, but I didn't bother to explore the level beforehand because of the time limit and then the issue of dozens of cops everywhere looking for me. I made a beeline for my equipment... and now I apparently can't finish the amulet of whatever.

Instead of restarting, I think I'm just going to quit and move on to Thief 3.

Thanks for the tips, someday I'll check out those other levels. I need to figure out how to configure DarkLoader for Thief 1.

I just finished Hammerite Imperium on Shadow difficulty (but really, why do it on anything else?).

Highly recommended.

I really want to try out the Thief 1 missions but I need to figure out the issue with Darkloader.