This would be awesome. For too long now has the PC community been endlessly waiting to finish the fight, and what better way than give them the ultimate halo 3 remaster? With microsoft finally noticing and accepting the PC community for once with the announcement of Halo 5 forge, and halo wars 2 coming to PC, as well as a whole bunch of xbox games being ported over, it would be a welcomed addition.
I have this crazy theory that what you are saying is absolutely going to happen and that in some way this will entail a redo of the MCC but on PC. Here is what I think, with the recent announcement that Halo Wars is getting a remake AND said remake is coming to PC which it never has been on before, this leads me to believe that Halo 3 Anniversary will be the same in terms of getting announced then having a collection of the other games alongside it come to PC. At this point we can all at least believe Halo 6 will come to PC as practically all the games shown at E3 were PC and Xbox and this new reform looks to be happening from here on out.
I can see Halo 3A coming to Xbox Play Anywhere, cross-platform between Xbox One and PC, would be awesome lol.