If SSB4 had any transforming character, and I'm sure it will, it would be Bowser Jr. in Shadow Mario.
I am almost positive that Bowser Jr. be the new Mario rep of SSB4, but I'm not sure about Shadow Mario. It seems like a 50/50 possibility to me, it just depends on Sakurai. Bowser Jr.'s major game role was in Super Mario Sunshine which makes it seem likely, but Shadow Mario hasn't been seen with Bowser Jr. for a while.
The transformation method is the same as Zelda/Shiek and the Pokemon Trainer. Down B. That way, the moveset for Shadow Mario will not be as difficult as it would if you had to make four different special moves. But here's my idea for the moveset:
SHADOW MARIO
Standard B: >GRAFFITI< ~ Shadow Mario swings his brush at enemies, resulting in a bit of graffiti flying off. The longer the move is held, the more graffiti will come off. If an opponent is hit with the Graffiti, they will take damage as long as the graffiti is on them (wares off after about 4 seconds). If the attack does not hit an opponent, it will land on the ground and opponents will trip if they walk in it (lasts for 8 seconds). To keep this from being spammable, it must charge up like R.O.B.'s move.
Side (Smash) B: >BRUSH DANCE< ~ Shadow Mario hops forward as he swings his brush at opponents. If the button is continually pressed, Shadow Mario will continue to inch forward, skillfully swinging his brush. If he connects with the fourth swing, opponents will lose balance and trip due to the slippery graffiti of the brush.
Up B: >BRUSH TWIRL< ~ Twirls brush quickly around himself resulting in hovering slightly above the ground. The faster the button is pressed, the higher Shadow Mario will rise. Better for horizontal recovery. Brush causes damage if it makes contact with opponents. *NOTE: This move was actually seen in Super Mario Sunshine, but without the Brush.
Down B: >BOWSER JR. TRANSFORMATION< ~ Obviously, Shadow Mario takes the form of Bowser Jr. and pulls up his mask, transforming him back into Bowser Jr. As seen in Super Mario Sunshine.