Actually what we have seen in the game was only a small part involving certain scenerios. I mean a full fledge type season with its own realm to play in, also a high powered FE story arc with its own rewards, and possibly marks or events.
Sure we have the occasional Stadi appearances and incurions - but there is much much more to offer for those with a Mirror Universe Hunger. Heck, may even involve the Tholians, they always doing time travel or time capture type missions.
Introduce a Bad@ss Mirror Universe Villian, perhaps a carbon copy of players main toon, that would be ingenius. To have the player combat themselves in a FE - The doppleganger.
When we got the Shard of possibilities the devs said it took new tech to get multiple copies of our toon on the screen at the same time. It would be crazy awesome if they could use that tech now to build a mirror universe version of our self.
Doesn't champions let you choose a Nemesis? It would be awesome if STO had something like that.