I tend to like games (and movies and such) to stand on their own. Even sequels I feel should be completely playable and understandable without ever playing previous entries. It is neat when things take place in the same "world," like say all the DnD games, but stuff like the Marvel movies make them way too interdependent.

That said if you made "Doom Guy" travel back in time and help BJ kill some fascists I wouldn't complain.

Bomberman vs. Pac-Man Choose to play as either title character and they each play the same way they always have. Pac-Man just wants to eat all the pellets; Bomberman is tasked with placing bombs in his path to stop him and has a points system based on how many pellets remain.

How about Chrono Trigger and Romancing SaGa?

Use Chrono Trigger's time travel setting, but Romancing SaGa's non-linear structure where you have a choice of main character. and your choice would affect the era you start in. Then, add SaGa Frontier's growth system, particularly for non-human characters (like robots, for example). Put in some double techs, and there could be an interesting game here.

DMC + Bayonetta? (Actually no: they're each an alternate universe version of the other..)

Imagine a crossover of Zelda: A Link to the Past and Super Metroid, with item placements randomized and shuffled between the two games.

Actually, you don't have to imagine this, because it actually exists.

Similarly, there's a Zelda 1 and Metroid 1 crossover randomizer.

Just Tropico Build up an island nation and then destroy it one settlement at a time with all the gadgets from Just Cause 3 or 4! (Does JC4 add anything? I hear nothing but bad.)

Shin Megami Tensei III: Nocturne, featuring Dante from the Devil May Cry™ Series.

What's the point though? Feels like pointless fan service.

Because many people like both games and find randomizers fun, so they like the idea of having to go between the two games, finding items in one game that are only useful in the other.