I'm not sure about all of them (some of them already had spirit battles tied to them, or in the case of characters like Heihachi or Travis Touchdown, were going to be included in the game as spirits later), but it would be nice to have more of an incentive to get Mii costumes.
Things like the Inkling's trophy in Smash 4, or the music that came with the Sans, Cuphead, and Shantae Mii costumes, definitely made those costumes stand out and made me more willing to get them. They could have added
At Doom's Gate with Doomguy, or some Bomberman music with the Bomberman Mii costume (I'm partial towards
the Super Bomberman boss theme)
Though for costumes like Vault Boy, Altair, Jacky Bryant, or Team Rocket Grunt, where no spirit events seem to be planned, it would have been nice to get a spirit, even if it was added to the regular rotation later - it doesn't seem like it would take much to add a single PNG, though a spirit battle might take a bit more work. Having a little incentive like that helps; I can't see a spirit being permanently tied to a costume (since they shouldn't lock stuff like that behind a paywall permanently - even the spirits that are tied to game releases have been added to the rotation eventually), but being able to get it without having to do the spirit battle or the post-battle mini-game would be a small bonus.
