Well, supposedly (and don't quote me on this), Sega and Nintendo's deal has been "extended", by no longer considering the latest Mario and Sonic game part of their three game deal. While, if true, it's not proof positive of a possible long-term business relationship between the two, it certainly is a step in that direction. Though Sega will never stop making games for every system, Sonic Team might become Nintendo's "step-child", making Sonic himself (as well as NiGHTS, Amigo, etc.), but not Sega, exclusive to the Big N. I'm not going to lie and say I wouldn't be ecstatic to hear of Miyamoto making a new NiGHTS (which he's expressed interest in doing).
Oh, and on-topic, I think Sonic is enough. If Sega were to get another character (which I find unlikely and unnecessary), I'd rather they be from a different franchise.