With the exception of Pac-man, I haven't had a personal connection to any of the other third-party characters in Smash, at least, not until after playing the Smash games they debuted in. I've played some of the Street Fighter and Marvel vs. Capcom games, so I'm not unfamiliar with Ryu, but I don't really care about Ken either way. Don't care whether Dante gets in or not either, to be honest.
The only third party characters that I'd be really happy to see at this point are Spyro, Rayman, and to a lesser extent, Crash. I don't see any of them happening soon.