It would be a little silly for them to re-release the multiplayer for Halo 3 and Halo 4, since they're both available on the 360 right now. Plus, releasing four games worth of mutliplayer hoppers would seriously fracture the (relatively small) player base for the XB1. You're talking 50,000 or fewer players for each game at launch.

I bet what we're going to get is Halo 2: Anniversary, with ports of the Xbox 360 versions of the Halo CE, Halo 3, and Halo 4 campaigns. So you'll be able to play the campaigns of all four games as well as play Halo 2 multiplayer.

Of course, if you really want to get into the weeds of speculation, consider this: What if these aren't really ports of CE, 3, and 4? What if Microsoft is launching backwards compatibility with either emulation or cloud streaming? They showed off Halo 4 on the Xbox One during a developer's conference last year, so it's a possibility.

Also, unlike today's Halo 5 news, this news isn't coming from 343 and Microsoft is declining to comment. I bet this is going to be officially announced at E3. With Microsoft releasing so much news (Price drop, Games With Gold, Sunset Overdrive, Halo 5) prior to E3, they're going to need some mega bombshells at their press conference.

Bombshells like backwards compatibility through cloud streaming.