You may consider adding Pharaoh/Cleopatra to the list. In some Cleopatra missions you are dealing a lot with Roman armies and ridiculous requests. Maybe it's not a big appearance but I think it'll do :)

Added. Thanks!

A few more to add:

Greek:
Indiana Jones and the Fate of Atlantis - a lot of references to Greek mythology/literature (famous Lost Dialogue of Plato)
Populous II: Trials of the Olympian Gods

Roman:
Celtic Kings: Rage of War
The Settlers 2, 3, 4 (I don't remember if later installments have Roman faction or not. Probably not)
EDIT: Praetorians

Invictus: In the Shadow of Olympus is an RTS/RPG that is based on Ancient Greek Mythology. Among other things, you can enlist mythical heroes like Achilles, Electra and Hercules.

+1 for the great idea, that I saw just now :(, btw do we have a way, something like a search function for GOG mixes? Maybe in MaGoG or Adalia. How do you check if your idea for a GOGmix isn't already realized by someone else?

Thanks guys! added the games.

You could add Planescape: Torment to your "Literature bonus content" mix, since it contains a (amateur, I think) novelization of the story drawn directly from the in-game dialogue.

Thanks, added.

Metroidvania

I created this GOGmix because current metroidvania (genre) does not include all metroidvanias. Plus, the other similar GOGmix (List of GOG Metroidvanias) is not updated for a long time. I hope my GOGmix will be useful for metroidvania fans. If you like it, send your suggestions via chat.

Games with New Game Plus mode

Stub. If you're interested in it, please help me to complete it (via chat). Thanks. :)

You could add The Witcher 3 who receive the new game+ mode in the free DLC.

Added it to GOGmix. Thanks.

Hard Reset Redux, which just release also have game + mode.

Just noticed this thread. Looks like a great idea for promoting some GOGMixes here. I've got one about games that have elements of time travel in them. It currently has 9 votes. I'd appreciate some votes to bump it a bit further up so people can see it when viewing a game with time travel.

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