For years now, SEGA has just been nothing but Sonic and Yakuza. I would like to think that if any SEGA franchise besides those two is to be considered "major," it's NiGHTS. Maybe Virtua Fighter. Super Monkey Ball maybe?
I'm a little wary on a potential NiGHTS 3 and will continue to be until I have reason to be confident in Sonic Team again. As long as they're not consistently putting out good Sonic games, they're not gonna be putting out good NiGHTS games even if they try; that's the lesson I learned from Journey of Dreams. It looks like Mania and Forces will be good, but so were Colors and Generations, and it took a while for a promising Sonic game to release after that.
I've got bad feeling that my definition of "major franchises" differs from SEGA's quite a bit. What I think SEGA is trying to do is introduce as many people as possible to older franchises with SEGA Forever, and then see about actually reviving some of them. I don't think this will be the case for all of them or even most of them, but maybe one or two. Given that the Spielberg controller has shown up in the teasers, I find it quite likely that NiD will see a SEGA Forever release; perhaps using the touch screen as a "Mindsight" of sorts.
I don't see any of those currently on the App Store via SEGA Forever getting a revival anytime soon, but we'll see. Wouldn't rule out Altered Beast nor Crazy Taxi. As far as I know, there hasn't been a SEGA Saturn game on SEGA Forever yet, so it's possible that NiGHTS will be the first.