I'd message you, but your privacy settings disallow this.

Anyway:

+ Infinity engine games - I think all Infinity engine games (as well as NWN) supported exporting your character(s), then starting a new game with them. I'd count this as a sort of DIY NG+, but at the very least, Icewind Dale should be there:
the Heart of Winter expansion pushed the level cap to 30, and a full playthrough (main + HoW) would still not get you to max level. Enter Heart of Fury mode - difficulty is locked to Insane, enemies get even more health, but killing them also gives more XP. The game does, however, allow you to start in HoF mode with new characters. Hey, it's your funeral.
+ various Diablo-likes - Also debatable, but these usually allow one to start again with the same character, often offering multiple difficulties. An example that specifically calls this NG+ would be Torchlight 2 - it allows one to go up to NG+5.
+ Bastion - NG+ unlocked after beating it once

Grey Aliens, Close Encounters, and UFOs

I just wrapped up this GOGmix listing games that include alien greys, flying saucers, and other such phenomena.

I'd definitely appreciate it if anyone knows of other games that might need to be added! Just contact me at the dedicated thread.

<span class="bold">Games not available on Steam</span>.

There was already a GOGmix for this, but it's dated.

My favorite mixes:
Games that have native linux ports...elsewhere by Redfern.
Lossless soundtracks by plagren
There is even a Demo!" :) by gamefood.

Maybe should combine?

May I suggest adding this series: VAMPIRE: THE MASQUERADE - Redemption and Bloodlines.

If you like RTS games have a look at:

Must have RTS

Why do people bother to make GOGmixes with no descriptions? For certain kinds of GOGmix, I can sort of understand, but for a personal recommendation mix, I'm not going to care about your list if you can't be bothered to write a brief note about WHY you recommend each game.
Just something that's bugged me for a long time.

I recently made such a mix. However, my intent was to come back and add a review to each of the games. Maybe, something in between would be a message such as this: Review coming soon...

It's one thing if you just made the mix, and wanted to add all the games first, and come back later to take care of the descriptions; but there are tons of GOGmixes that have been around for months or years, and which never got any explanatory text added after their initial creation.
I mean, I can understand forgetting one even made a GOGmix -- they're really not very well integrated into the rest of the site, and even the mix's creator would have to resort to bookmarking it in their browser to keep track of it. But there are so many pointless GOGmixes out there (and high-rated but horribly-out-of-date ones, too).
It's one of the reasons why I've never created a GOGmix myself -- I don't want to have it floating around out there when I inevitably abandon it or forget about it.

I know what you mean. I've seen those. I've also seen ones with free games and they are from 2011. It's true. You have to take responsibility for your mixes. Speaking of which. Is there a way you could view your own mixes? I've seemed to have lost mine or it got deleted somehow.

That's what I meant by "even the mix's creator would have to resort to bookmarking it in their browser to keep track of it" -- otherwise, you'll have to resort to a Google (or whatever) search, or going to the product page of a game you know is in the mix, and trying to find the mix in the sidebar there (this is only reliable when trying to find already-popular mixes).
Do you remember what you called the GOGmix? If so, just search "[title or title fragment] site:gog.com/mix" (without quotes, and obviously with your mix's title replacing the bracketed placeholder text).

Ah. I see. I've tried that. Grr. Yes, I will resort to Google.

Mix updated with some reviews:

Must have RTS

Created a new Gogmix for the best immersive sim games on gog. Feel free to PM any suggestions for more great immersive sim games or games like them and I'll consider adding them to the gogmix. :)

Don't exactly know what an immersive sim is? Then please watch this video.

https://www.gog.com/mix/best_immersive_sim_games_on_gog