Just pretending all sonic games were on nintendo consoles and the series was 1st party, this is what we'd have in each smash, assuming we kept the same amount of characters:
SSB64: Ness, Jigglypuff or Luigi are cut while Sonic gets in. Yoshi is bumped down to unlockable.
Melee: We get Tails over some other unique character. Ice Climbers, Mewtwo, or Peach are decent contenders. Peach isn't as popular or as important as Wario, I'm still amazed she got in before him.
Brawl: Shadow gets in over Wolf due to the series being far more popular. Shadow would resemble Sonic slightly, but barely, in the same way Wolf does to Fox. Why Shadow? Because he could at least be vaguely similiar to Sonic, reusing a couple bits of work, which Sakurai's team would be overjoyed at.
My point? Sonic's an important series, and it's only being given this status due to it's 3rd party ranking. There really are no other series that deserve a second character before Sonic, and the Sonic series and it's characters are far more iconic then a decen tamount of the characters in Brawl. Of course, it's not Nintendo, which is why it gets lowered, but when they start having to pull character choices out of things like a fourth Starfox, DK or Kirby Rep, the important 3rd party characters will simply be unable to be ignored. SSB4 can still get away without too much 3rd party emphasis, as there are still enough characters to add that are iconic and popular enough, but SSB5 will be terrible without more 3rd parties. SSB4 certainly could use more 3rd parties though. It doesn't have to be a big jump, just three or four, and then SSB5 can jump up to bigger numbers.
But I don't think it'll happen. They don't want to advertise the games of their opponnents. Sonic fans will already buy it for Sonic. Adding Tails, Knuckles or Shadow won't really give that many more sales.