Making ToO a Mercury-only event = a good idea. I like that theming. It gives the activity a sense of uniqueness.

Only one map = a bad idea. Can one map really sustain an entire mode?

They should have more Mercury maps, but it isn't happening, at least not in this "expansion".

Hey, long time no see!

Dota 2 does fine with one map. Burning Shrine is also by far the most interesting for skirmish. Destiny has already shown staying power with single events in a limited location (raids), I don't really see an issue with Burning Shrine for all Osiris games. The variety in crucible come from weapon variety, and if you want to take issue with the limitations and obstructions Bungie's recently put up to that, that's a different subject.