For the next Halo game, I believe they will go the traditional route with its own campaign, multiplayer, and so on. I doubt they will go F2P again except for a BR mode. Basically, go the CoD route. I would rather 343 stick with the Slipspace engine and keep updating it.
We might not see a new Halo game from 343I for some time, though I think a spinoff is coming before that. Wasn't there a rumor of a spinoff Halo game in the works?