A comment down a bit has me thinking about what gameplay lends itself the best to a wacky crossover fest, Like which gameplay style allows not only for as many characters and series to join, but which style also allows each series to have fanservice and a time to shine as well as a Narrative that binds it all together.
These are ranked from Most Crossover Potential to Least Crossover Potential and I will try to add a specific example where applicable
1. JRPG (I think JRPGs like Project X Zone and Super Robot Wars shows that this is the best genre for Crossovers, these games specifically not only have a lot of fanservice and characters but properly weaves them into each other giving fantastic interactions through dialogue which very few of the other styles can truly replicate)
2. Fighting Game (Do I even need to explain this? This includes Platform Fighters)
3. Moba (Mobas get a bad rap but man can they lend themselves to some fun crossovers, Heroes Of The Storm is my go-to example, Not only do you see fun interactions between different characters, but also original things that put the characters in interesting scenarios)
4. Racing Game (Another self explanatory one, CTR is probably the best one for this category)
5. Musou/Warriors (Self Explanatory)
6. Shooter (I feel the Potential here is massive but It hasn't really been done all that much, Overwatch, Fortnite, Cod and paladins have crossover skins but there hasn't been a proper Crossover Hero Shooter as far as I know. There is Dead By Daylight)
7. Sports Games (These can get real wacky, but I also feel like there is only so much you can do, The Mario and Sonic olympic games are probably the best example of what a crossover sports game could be
8. Platformer (Another one that needs some proving, Crossover Platformers could mix gameplay and styles and make things fun. but is really limited with how many series can crossover. Sonic Generations counts, I think.)
9. Rhythm Game (This heavily depends on the visuals of the game, But as fun as seeing all these music tracks next to each other they don't interact. Pop'N Music and Taiko are my 2 examples)
I realize after typing this that the glue that makes some fun crossovers is actually the original stuff added in that binds the series crossing over.
Or maybe they still would, but for a different reason