Dry Bowser could totally work as an alternate for Bowser, that just changes his properties. Dry Bowser I imagine would be a bit lighter so farther knockback, and his Side B, Up B and Down B would release bones opponents could throw at each other. Would it be possible for the bones to be like food for Bowser, but throwing objects for everybody else?
As for normal costumes, he'd probably keep his black color swap (Melee's Adventure Mode), his Red color swap (I dunno, Spiny as in the Koopas a Lakitu throws?) and his blue color swap (Japanese Super Mario Bros cover or Blue Spiny Shell from Mario Kart). I think they'll replace the brown color swap (Boom Boom) with [collapse= Spoilers for Mario and Luigi series] the blue Dark Bowser from Bowser's Inside Story as his main blue color instead of the other [/collapse] and then have his white color swap be replaced with the Dry Bowser alt. since I don't think there should be any real color swaps for Dry Bowser.