Can't wait for at least first couple to hit Steam. Properly ported Halo 1&2 .. with mods would be absolutely amazing. Heck, I'd take mouse/keyboard on Xbone for SP as well.
Reach is honestly going to knock a lot of stuff off the list, Match Making, Crossplay between Win10/Steam, Mouse aim (they will probably use a universal solution imo), and a lot of other stuff. The next biggest hurdle is probably going to be Forge and theater.
Halo 3 is going to take some time, they are reworking forge and theater controls and then applying those changes to Reach and 2A (which wont have forge at launch on PC). They might be adding some extras in there too so it might take just a tiny bit lol