We've been playing iM@S 2 for 3 years before One For All was there, and the first console game was released 8 years ago (4 years before iM@S 2.) So the gap between console games is like 3 or 4 years... so base on this roughly calculated schedule, I don't think the new PS4 title will come that soon.
Maybe there'll be a iM@S game for PSVita or another PSP spin-off. For next main game we'll probably have to wait until early 2017, even later since we're transiting to a completely new console (just like it took one more year for the dual-consoled iM@S 2,) but that's just a guess.
About Third Vision... Jesus, that will involve A LOT of work, bare in mind even if it's only Million Live, there're not just 13 of them... just imagine you're going to make a ML console game like the previous 3 for the original 765Pro, and there will be 37 girls to work with; we'll have 50 of them if you want to include the old ones.
Moved here to be more on-topic. I agree that it might not be out for a while, but at least a teaser for the game at this year's Tokyo Game Show seems likely for me since they probably have some things down for the game.
PS Vita spin-off is inevitable. As for if it'll be a producing game, I doubt it. It'll probably be another rhythm game ala Shiny Festa.
In terms of Third Vision, a roster expansion is inevitable. However, I don't think they're going to add ALL the Million Live idols. That'd be too many routes. I'd say they'll probably add four or five of them and they'll go for either the most popular or the ones with the best story potential, like Anna, Konomi, Shiho, or Momoko. The most exciting thing for a potential Third Vision are the possible character redesigns.
