Some things don't go well together. Like peanut butter and soap, or open PvP in a neutral quest hub, or melee attacks for a spellcaster. On the other hand, there are some things that seem as if they'd go together so perfectly that it's a wonder it hasn't already been put into place. Final Fantasy XI's multiple jobs paired with the unique nature of Warhammer Online's classes, for instance. Or the skill-based approach of Guild Wars in a classless game such as Fallen Earth. Or Sword of the New World's multi-character system combined with, well, more or less anything.
When you've played enough games, you start seeing how systems could be stapled together to produce something very different in the end, and possibly even something better than either system individually. What systems would you like to see combined? Would you want them to be in a current game, a new game, or in a sadly-deceased game that would have been uncommonly excellent with the two things put together? Or would you not even look at systems, and just like to port the engine of one game (such as the gameplay of City of Heroes) into a totally different setting?