Halo 3 Anniversary coming next year!
Seriously though, I do think H3A is coming next year and it'll be on XBO and also PC.
Or there is going to be some type of spin off (similar to ODST) for Halo 5.
I'd love this... I think it would be a decent opportunity to make some sense of Halo 5's story.
Story spoilers:
I read something today that suggested there was a forerunner alive on Genesis during the events of Halo 5, and he was behind the scenes helping Chief and crew work against Cortana. This is exactly the type of thing that should have been presented in game, instead of locked behind audio logs... Of course it also would have meant more time w/ Blue team and less with Osiris... Can't be mad at that. Currently whatever the hell was happening on Genesis sounds like good ODST-style DLC material.