I totally believe Doc has a chance (The level of chance I'm referring to is that of greater than Ridley but less than that of Krystal) to come in as an alt, they only reveled 4 alt costumes for Mario and if you include his original theres only 5 so the 6th costume could be Doc and maybe they'll do a few bonuses like they did for Wario (Here's hoping one of them is black)
The only issue is the obvious Mario Fireball issue and even the FLUDD to certain degree (but maybe you can overlook that) BUT If Mr.Sakurai cared enough in the first place to put him in Melee I'm pretty sure he's willin' to at least change the look of the fireball (Not the physics).
Also the new Dr.Mario Wiiware game isn't bad for Doc's chances either