I have fantasized about creating a videogame where there is a campaign mode. In this campaign mode the player has to beat the AI in tens of different chessgames and every game has a different variant of the rules (regular chess is included too).

After you win all the games the cycle starts again but the AI each time is smarter . The replayability value of a videogame like this would be limitless but you have to find a good engine to support all the variants I suppose . If you manage to create something like this who knows ... it could even sell

Well, I don't know which variants you had in mind, but Fairy-Max comes with pre-defined variants:

FIDE

Makruk (Thai Chess)

Seirawan Chess (B+N and R+N compunds introduced during opening)

Shatranj (ancient Arabic Chess)

Knightmate (K moves as N, N as K)

Capablanca (10x8)

Gothic (10x8)

Courier (medieval variant on 12x8)

Cylinder (h-file connects to a-file)

Berolina (Pawns capture straight, move diagonal)

Great Shatranj (10x8 without any sliders)

Spartan (blackplays with 2 Kings and Pieces / Pawns with non-standard moves)

Chess with Different Armies (all combinations of FIDE, Colorbound Clobberers and Nutty Knights)

Fairy-Max is open-source freeware, and the engine proper is only about 100 lines of C code, (plus a file with game definitions of similar size), so you could easily integrate it into another game. And you could have it play by an ever increasing number of nodes in its search tree to crank up the strength.

Thank you, that was more helpful than several hours searching on the internet. I also like the campaign game idea, but perhaps also having individual chess variant campaigns would improve it. I really like that idea. As for the variants I am using, mostly ones that are not already made into programs. Most of the variants will likely come from The Classified Encyclopedia of Chess Variants. I highly recommend the book to anyone who is interested in and enjoys chess variants. Suggestions and new games are welcome and thank you.

 I concur.