"An interview between Iizuka and Famitsu lists a end of February release for the game in Japan. From what we can understand from the interview, characters will run forward on their own and Eggman will make appearances during stages which there appears to be 50 of at first (if we are reading the translation correctly). Tails and Knuckles are also playable, but Sonic is the only character initially available. Famitsu also went hands-on with the game, mentioning a cutscene featuring Sonic, Tails, and Eggman. Jumping appears to vary based on where you tap on the screen, possibly indicating that you tap on enemies to homing attack. The Eggman boss battle appears to have Sonic throwing or knocking back Eggman’s projectiles to defeat him."
http://segabits.com/blog/2015/02/02/sega-releases-the-first-sonic-runners-teaser-trailer/How 'free-to-play' can work for this title and I would be OK with. Base game has all the levels that span Sonic's history, multiple paths and ways to play. Leaderboards are world wide and new content would be added after awhile. Instead of just going around and saying "Christmas Sonic" and crap like that, I think they should really look into the history of the franchise and pull some really obscure stuff to add on to the premium level.
1. Don't force people to pay to play. Sorry, not good.
2. Skins (like make Sonic look like Sonic R Saturn Sonic), items and such are paid to get.
3. Allow players to win premium items without paying: Like digital in game coins after you win races goes to a slot machine where you can win premium items. More you play, the more chances.
4. Try to get sponsored events online and people that participate get the content for free.
5. Buying Sonic toys, console games and such all have unique codes to unlock items that deal with that game. This could be cool for Sonic Archie readers, lots of cross promotion can be done.
So what will happen? Probably none of the above.