The Halo fanbase is dwindling, reverting, and will die without rebuilding the fanbase, especially if Halo 5 flops.
Right now, halo 3 is reaching just as high numbers as Halo 4 at certain times, which is a dire state of affairs. However, as okay a game that halo 3 was, many people complained about a lot of the mechanics, however now a days its a gold plated turd compared to the standards of today's Halo. In fact, unanimously the Halo 3 beta (for those who played it) is considered the best build of halo ever. But that's for another time.
You have to ask why Halo 3 is flourishing right now like it is. It's because fans want Halo to be Halo. The mechanics have changed, forcing everything else to change with them (like the maps, ugh..) but as the online numbers show, it's not what works for Halo. We were hyped before release, we wanted to give it a chance but the more we played the more we realise, it isn't Halo.
Halo's biggest problem, and one of it's strongest attributes is the diversification of the fanbase. However with recent releases it seems apparent that particular groups of fans were being shortchanged.
Remake Halo 2 in hd, but change nothing else, release all the maps and make it post patch (please no 6 beatdowns for a kill). Halo 2 was what built the foundations for xbox live, and has the most to offer with some of the best maps that weren't rebuilt in janky, geometric, ugly one colour blocky forge rebuilds.
Releasing H2 will bring back a lot of people who have left, and will also help rebuild the franchise.