IGN did an amazing job in their new massive interview with Ono san where he reveals quite a few AWESOME news.
IGN: Will there be a new theme song for Super IV?
Ono: Yeah you don't have to worry about being indestructible anymore, haha. We're fixing that. Speaking of lessons learned, when looking at something like this for the first time your assumptions might be that this is some kind of downloadable pack or something, and to be honest with you that's not going to be. There's just too much going on with this release and too much content.
IGN: Now as the progression of Street Fighter carried on in the past – Street Fighter II, Turbo, Super – we'd see the same characters return but with different moves added in. You'd see tweaked sets, or new moves like Ryu's half circle fireball… have you explored any of that for Super IV as well for the existing characters?
Different or changed rival/pre-match dialog.
Along with new Ultras and tweaks, older characters will be getting new specials or normals.
Indestructible will be gone. Ono has “learned his lesson”
As the final boss, people who hate Seth, will hate him even more now. He will be harder but not, “unfair hard.”
New stages will certainly be added, but certain older ones will receive new elements or details.
No fighters will be locked, everyone will be accessible from the start.
No form of World Tour mode from Alpha 3 planned anytime soon. If enough people request it for a sequel, it's possible.
Frame data has been changed on certain character's FA, to better balance out certain things. The FA system will NOT see any major changes.