Random take, but I think it'd be cool to see more games include things like Missions or Event Matches or whatnot. One of my favourite elements of games like Sonic and SEGA All-Stars Racing or Super Smash Bros. Brawl was these fun bite-sized single-player morsels of unique gameplay, often with unique elements or requirements that let the game showcase different characters, mechanics, or otherwise in a fun way.
Taking Brawl as an example, having most of the Event Matches require you to play a certain character works well as a way of having the player get introduced to characters they might not otherwise play, as well as including funky tongue-in-cheek references (which I guess Spirit Battles are the ultimate evolution of?)
It'd be cool to see that kind of thing in more crossover games, given it's a fairly simple way of bolstering single-player content without too much extraneous stuff (like cutscenes, new mechanics or stages, etc) whilst still allowing for some interesting references and potentially a lot of fun for single-player content!