Please please please I cannot stress enough how much more 343 needs to give control to the PLAYERS.
Halo 3 was in no way the best halo, but I played it faaar more than any other halo, because of the forge & custom games. Forge was brand new, shiny, and had that new car smell, and it had everybody excited. I was so excited for the improvements to it with reach, and hardly any happened. However, they did improve, and it worked great! Halo 4 came, I was expecting more minor improvements. NONE. Actually got downgraded with the lack of a formal infection game type as it was, among other things. Halo 3 was one of the most hyped games of all time BECAUSE OF FORGE. Remember that? Allowing the players to use their imagination keeps the game far more interesting for far longer.
The single best thing you can do, is make sure that players can do LOTS with forge, and make it easier for them to connect with other custom games users. Try to keep forge simple, but let us go as far as we have the time to go!
So how can you do this?
1) the simplest part; bring back more custom games options! The setup now doesn't even allow for half of the old infection variants we used to enjoy! Add more options! Let the infected players have more options, & in general just add every freaking option you can think of.
2) the (possibly) hardest part; COMPUTER PLAYERS. This always makes me think back to my days playing on a wow all GM private server. Let us put down enemies or friendlies! Let people spawn characters such as elites, customize them, give them an allegiance number. (Ex. 1 is spartan's side, 2 is covenant, 3 is forerunner. 4 would be nuetral to all 3, you attack it & it attacks you, but otherwise it's just there. Combinations of numbers would make them friendly to multiple teams)
3) the (possibly) medium part; you've probably got this a lot, but there's 3 parts to part 3:
1: More objects! Halo 4 had such a small selection of objects. Just that! More stuff! Rocks, trees, all sizes of blocks, bunches of buildings, towers, etc
2: TERRAIN EDITING! Just simple terrain editing. Make hills, flatten areas, make holes.
3: More objects, in a different sense. Let us spawn more of one object type. Saying "oh you only get 100 building blocks" limits things so much. And included in this, higher budgets! Yet again, all of what I'm asking here is more options and less restraints
If any of this advice is taken, I'm sure you'll have me playing halo 5 for years to come. Because I played halo 4 for about 50 levels in multiplayer and one single player play through, made a couple forge maps, and got tired of it and went back to reach & halo 3.
Nice job on everything other than forge though