There's no roster, just an analysis of possible characters, with five being fully decided upon. The five are Debu (a new character from the upcoming Yoshi's Land), Ridley, Little Mac, Takamaru, and K. Rool.
In all honesty, give the huge popularity, sales, and overall fanbase of the Smash Bros series, I think any publisher would have to either be complete idiots or would have to actively dislike the idea of making easy money to not allow their characters to be used in Smash Bros if Nintendo/Sakurai comes to them with the request. Beyond whatever monetary amount they would receive from Nintendo, there would also be the prestige, future sales, and overall good p.r. that came from the character being used in such a huge series.
Seriously, anyone who would turn Nintendo down for Smash Bros characters is too stupid to ever be allowed to make another video game decision ever again.
My guess would be that negotiations would involve more than:
Sakurai: "I'd like to add Megaman to the Smash roster."
Capcom: "No. We hate Megaman."
Capcom's response could certainly include something along the lines of:
"We were hoping you'd consider Ryu instead -- we think it might boost sales of SFIV3DS and our upcoming SF release on the WiiU."
I still believe Megaman's chances exist (for many of the reasons that have already been discussed), but an oversimplification of the process of negotiations isn't a real argument, IMO.
Series that desperately need another game: LoZ:FS, BoF, Chrono and MM:L (You were so close, my friend!)