Namco Bandai are clearly going to get a representative of their own since they are helping co-develop the upcoming game for the Wii U console. My likely choice for this is Pac Man but thats just me. I would never rule the possibilities of a Tekken or Soul Calibur representative thrown in the mix instead to attempt to shake things up but I personally just don't see it.
I would also really dig a Sonic return to the franchise as not only am I huge fan of the character but Sega and Nintendo have been the closest in terms of working together that they have ever been. Sonic returning to Smash is still debatable as his appearance in Brawl last time could have been a one-off deal but leaving Sonic out of the Smash Brothers picture entirely to me seems like just another wasted opportunity.
Last but not least, maybe least in terms of the probability of this actually happening is Capcom having a rep in the upcoming mascot brawler. We all want to see Mega Man, he is our guy not only to represent Capcom but as the ultimately desired third party character choice as well. His treatment as of late though with Capcom's intent on burying him he seems the least likely to combat out of their stars. An addition to the roster in the form of a Viewtiful Joe or a Street Fighter character like Ryu seem more likely as brand name fighter that Capcom could potentially throw in the mix of things, if they even do so at all. For more on my opinions on the subject in this article please watch the video below.