A custom template feature - in h5 you can group objects to move as a single piece, can we have an option to save those to a custom template so if we want to use it again on a different map/build we can just pull it out the menu?
Covenant structures - most set pieces like bridges, walls, prefabs etc. Always seem to be human/forerunner, can we get some covenant version to?
Map skins - they don't change the geometry of the map but you could change the ground between grass, grass/snow, snow, grass/sand, sand, glass, volcanic basically any type of ground they can think of. You should also be able to set the colour of the rocks separately... Think like if the cliffs and rocks on coagulation from halo 2 had been more grey than orange and the cliffs on containment in h2 were more orange than grey. You could make forges feel more unique that way. But regardless map skins should be a thing.
Skybox/Tod skins - this speaks for itself
AI - basically you pick your AI unit, could be a boss, could just be a squad of grunts... Set their equipment and difficulty level, then place a volume (cube, cylinder, sphere, whatever) that they can spawn in and not leave. Set if they drop weapons or not and BOOM... you now have a camo sword elite that drops a full power sword on death in that room rather that a random sword on the floor, interesting power weapon spawns! You could set if they're worth kills for slayer, or if they have a team alignment for objectives, have a handful of marines or something guarding your base. Or some prometheans guarding the hill in a king game. Ooo new idea! You could set them As the objective! So instead of running a flag you escort a prisoner or instead of planting a bomb you kill a vip. (yes I write like I'm talking, it's more fun and helps get my ideas out...)
Ignore my first one, went on forge for the first time in ages last night... That's a thing now apparently