Street Rod!
Those games were awesome, they could do something a bit more modern given the popularity of NFS games these days.
Gearhead Garage was ok but had no racing :(

Ugh, I wanted to avoid it, but what I really want is a sequal to Unreal - unfortunately there has already been one, problem is it sucked. A sequal should have you battling Skaarj and face hacking Pupae (pre headcrabs anyone?) and saving lanky four armed creatures that really like chanting and showing you hidden rooms...

I would love to see a "real" KOTOR 3 game. MMO's just don't do it for me.
Going back to an older series though..Wing Commander. Why oh why can't this series return.
X-wing too for that matter.

There are SO MANY of them. And at the same time, everytime I hear about game sequel/remake I fear so much. I bet you all undestand. I spent my childhood and teen years playing them and I love many of them dearly. Seeing bad sequel is like seeing old friend taking hard drugs...
But anyway, if we are are talking about possible good sequel I would currently vote for Vampire the Masquarade: Bloodlines (I know it is not so old game, but I loved it so much). I would love to see that game in episodic style publishing.
Of course, I love Troika games (R.I.P.), Black Isle Studios (R.I.P.), Origin (R.I.P.) and many many more (thanks god there is Bioware,... and maybe Bethesda - I did not like their TES series, but I am ver curoius about Fallout 3 - on of the best RPGs ever).
It is really hard to pick just one (eventhough I managed to do that). I would really love to see much of my old friends again, with another story and same feeling.
I was also thinking about Planescape Torment and sequel, since it is my game number one - clearly. But I think the original story is irreplaceable and the ending (eventhough open) is ending of the Planescape Torment story as is - the mortality which follows is more of redemption. But I would love to see some Planescape ruleset based RPG. Please give us more epic RPGs with flawless storyline, many choices and perfect atmosphere.
Oh, and Magic Carpet would be nice! With all the possibilities of destructible terrain and multiplayer (maybe even MMO?). Was a great game, played the second one for weeks (the spiders and zombies were vere depressive though).

DESCENT 4!!!!!
I can't believe that that has only been mentioned in passing! Descent deserves another sequel so bad.
I would also love to see FreeSpace 3. In the years since Freespace 2 came out, there is STILL not a space sim out there that compares to it.

I really miss the old MechWarrior games. They had their flaws, but the feeling of controlling a 100-ton 'mech bristling with guns and missiles is one I remember with fondness.
A Freespace 3 wouldn't be amiss either. One could hope that'll be Volitions next project after finishing Red Faction 3.
One game I've discovered through GOG was Hostile Waters. I really enjoyed it, and I can see a great potential for a sequel.
Hostile Waters reminds me of Battlezone II, which is possibly the best combination of TPS and RTS I've ever played. So I'd really like to see a Battlezone III game.

Dungeon keeper 3. I always loved 2. I thought it deserved a sequel.

Planet's Edge A sleeping wonder that is a great game, it ended on a cliffhanger (the earth disappeared). It really really needs a redo.
I also agree with those that said the mechwarrior and battletech series. I loved the original and Mechwarrior 2: Mercenaries.

Amen sir, amen.
Also, how about a sequel to Tyrian? Anyone enjoy local co-op as much as I did on that one?

I would have loved to play Journey to the Centre of Arcanum and a continuation of the SiN series.

Good question!
Freelancer 2: More of the same... sadly Microsoft will never release a sequel.. Sadly it seems the Space Fight Sim is an almost dead genre.
Freespace 3: See above! It would be amazing with todays tech.
Dungeon Keeper 3: Another game we'll never see, which is a shame as the games are undoubtedly 2 of the best games I have ever played.
Pizza Tycoon 2: See others posts regarding my obsession with this game... The sequels that did come out sucked... I want a proper update.
Space Quest 7: Cancelled by Sierra... Darn them... This would have been amazing.. I could play these titles forever!
Pikmin 3: More than likely to happen... Can't wait for this... excellent puzzler/adventure
Theme Hospital 2: Again, never gonna happen... Up the graphics, keep the core gameplay intact.. it would be amazing... I'm at least hoping the original gets a DS release!
Powermonger 2: I loved the original so much... But tried going back to it.. couldn't hack it.. Squinting trying to see anything.. A remake would do!

It's a goddamn shame Outcast doesn't have a sequel.

Hmm, let's see...
Max Payne
Grim Fandango
Secret of Monkey Island (one that's better than Escape from Monkey Island)
Psychonauts
Wing Commander
X-Wing Alliance

There's always the X3: Reunion. I haven't played the first games but I thoroughly enjoyed this... I'm really bad at it but still fun. And the graphics are incredible for the most part. There is a LOT of trial and error though. I found the manual a joke - in fact they re-released the manual online for registered owners. I never finished the story mode but made my millions by owing lots of stations.
There is also an Unofficial Sequel for Freelancer being planned as a TC. There is not a lot of progress yet but I really hope this pans out.
Karl

Deus Ex: Not an old game, but the second wasn't any good compared to the first. Fortunately it seems they have began to work on the third game. I really hope it is at least as good as the first one.
Theme Hospital: I'm not "anti-EA", but why did they have to buy/dissolve Bullfrog? Theme Hospital is probably the best tycoon/simulation game I've ever played.
Strike Series: I really wish EA revived this helicopter series, I just loved all its games. Played Jungle Strike a LOT!