(If you want, you can jump straight to shovel knight as the game appears on arcades)

The therapist's office

The king scepter (maybe even the shovel)

Shovel Knight in an assist trophy (You can also skip straight to final fantasy, monster hunter, fire emblem, persona, or indirectly to marvel vs capcom from here, but I'm lengthing the list for comedic effect)

Sora

I mean, pick your favourite

Cloud costume

Creatures from Final Fantasy XIV

Aiden appears in both MH (needs verifying)

Marth's armour (as DLC in MH and used by Lucina)

Alternate universe FE:A

Joker DLC in TMS#FE

Parody movie of the Avengers in Persona 5

I mean, it is the crossover IP of the last decade

Playable in UMvC3