One new costume for each character, coatless and shirtless alts for the defaults for all three characters, this stuff should be in the base game, I'm almost sure the coatless and shirtless alts will be available for free thanks to the modding community on PC, but they should be on console too. I don't like the idea of DLC Devil Arms either, even if they're returnees, just give them to Trish or Lucia if they ever become playable, because the idea of locking Devil Arms for Dante as DLCs is never going to fly well.
Season Pass should be a lengthy Vergil campaign with his own levels, bosses and enemies, and a campaign for Trish would be cool, but Vergil should always be the priority, and it should contain newer costumes for Nero, V and Dante, and maybe a bunch of new selectable backgrounds for Blood Palace, then a year or two later they can repackage the game with the season pass on disc as a Special Edition.
Also, instead of offering their own color palette alts, they should give players the option to adjust the colors to their liking.