Assassin's Creed IV: Goddess Black Heart.

...well, both AC4:BF and HDN:GBH have black in their name, but that's about it.

FarCry 4 and Dying Light. That would be awesome actually

Gran Turismo 6 and Kerbal Space Program.

I can't think of even the slightest idea of such a cross-over... My mind is completely blank...

Monster Hunter 4 and Resident Evil Revelations 2. Looks like I just create Dino Crisis.

Yakuza 4 and world of warcraft...

O.o

Strangely alluring!

Instead of a city, you would commit your crimes on the galactic level.  Instead of stealing cars, you steal Gummi Ships (or BBS equivalent).  Instead of guns, you cast magic and bash people with keys.  Instead of the cops chasing you, it would be Merlin.

OlliOlli 2 and Dark Souls

fittting, because the death count in both games is very comparable :P

I guess you get to do some skateboard tricks in front of a very ill tempered audience of demons and dragons that murder the shit out of you if you fail. All you can do is praise the sun.

this situation might be close:

Starcraft 2 and Fire Emblem: Awakening.

...Tactical RPG/RTS hybrid? Jeez it would take hours to complete a single rushed 5 minute game in SC2. Oh and story wouldn't really make a difference really.

Final Fantasy XIII and Pokémon Shuffle.

I guess that gives me Puzzle & Dragons? How anti-climactic.

DriveClub + Grim Fandango. Glottis likes to race, race through the land of the dead. Solve puzzles to unlock new cars. Have lengthy conversations with your sat nav Eva.