I always wanted a Splinter Cell vs Metal Gear crossover. The futuristic spy/war/mecha stuff from the first metal gear games meets the ground 'realistic' gameplay of the first Splinter Cell games.
It's a shame the story got total garbage with time. You started to feel the crap seeping in in a lot of parts of MGS2, MGS3 kept it under control but it fucking exploded in a bukkake of non-sense with MGS4
MGSV was such a steep in the good direction, I enjoyed the basic storyline even if there was a lot of

in the second part of the game.