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.

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