Can I do franchises as well?
Gran Turismo + RFactor = the best looking, most realistic racing game/simulator with the biggest car list when it ships, the best modding potential of any racing game by a mile, top shelf Photoshop car skinning support, real time telemetry that F1 teams would use...basically everything good about racing games, PC games, simulators, etc all rolled into one. Would probably be used as a testing sim by everyong with a car who takes it to the track, and would probably kill the recing genre because nobody else would release anything after THAT.
Grand Theft Auto + Yakuza = Brucie in Tokyo. Do I really have to explain how this would go down? (You play as Brucie, of course) Bonus points if all the other remaining characters from GTAIV get killed of in the opening cutscene.
Assasin's Creed 2 + that Japanese ninja game for the PS2 where you backhand your katana and fight bears and infiltrate castles + Splinter Cell = stealth AND free-running ninja videogame of solid win. The characters from assasin's creed are nimble enough (apart from Renaissance Batman, who feels like he put on a few pounds), and with a better stealth mechanic and some decent fighting and swordplay you could be onto a winner.
Ace Combat + Red Bull = WHY HASN'T ANYONE DONE THIS YET; Red Bull Air Race: The Game. You've all seen it on TV, now with some delicious next-gen graphics and a well-done cockpit cam and physics, it's be a wonderful addition to any gamers collection. Keep it updated with good quality DLC and you're set. (BTW, it's my idea, any of you steal it and there'll be trouble).
Damn I'm good.