I think the foundry should have every armor available in game and maybe even in the cash shop.
it can't be that we STILL don't have the various armor of hunter ranger and warlock, and now we'll get the paladin and probably won't get the armor in the foundry either.
There are also several free costumes there should be available in the foundry. for instance the ones from Icewind dale, the fashion from tyranny of dragons, etc etc.
Also we need all enemies that are not available as costumes. mainly non humanoids, gelatinous cubes, and bosses, even if it's only as costumes and not as real boss encounters.
We also are missing several sky backgrounds, like the well of dragons one.
This shouldn't need any extensive coding.
edit: I won't post any specific asset missing since there's already another topic about that.