Since they've intentionally kept the room hidden, it could be anything really. I'm assuming these rumors about there being 2 small chests and a single large one. Still, I think it'd be cool--since it's called a 'Treasure' room--if the room actually has a large number of chests in it, and Guardians just run around and pick a chest. Still, that would actually be annoying since that would actually promote the key farming. Ideally, since it's possible to get keys now, the original rumor of there being three chests is true. This would mean that, under normal circumstances, you can't get multiple chests unless a partner opts out of taking their's--which would make no sense unless they already did it with that character for the week and no keys drop for players who've already done it. Otherwise, the player would have to do it alone, which I figure is possible, but probably incredibly difficult, or just run with a single other person who doesn't have extra keys, thus they can't argue with you about taking it.

I reckon its a room filled with chests and you have to pick one with the key, all with different rewards.

It's a guaranteed exotic chest

I perfer the double d chest

I'll be going for the big chest

I bet there are hookers