blarg, bow chicka wow wow, boomshakalaka,
here are some ideas for reach, not all of tham are original:
CAMPAIGN:
not much but Elites, a capture building/ship/street & hold your position level and more skulls and more open world levels.
MULTIPLAYER:
Who remembers the last level of Halo 2 (Uprising). you know, the bit where you kill the big fat chief monkey, well i remade that in the sky bubble and its awesome (Most credit goes to my friend who built the large quantity of it). its on my file share if you have different cover weapon ideas plz edit. also the ability to turn some skulls on in custom games. i.e black eye.
FORGE:
just make the snap lock idea, the ability to make twoblocks one big one/ corner block and to be able to clone objects.
FIREFIGHT:
Assuming its still there. my friends idea (the same one who made the Uprising level) is that we should forge money for each kill, and between rounds, forge objects and weapons with the money you earned.