Well, DDO has breached the crystal shell, and traveled between worlds. It has opened up Epic content. Mighty powers have arisen, and there looks to be more in the future.

So I'm suggesting another planar breach, one easily supported by lore. One that is rife with both roleplaying opportunities, almost any gear and spells the devs wish to include, and near-infinite possibilities.

Sigil: The City of Doors. The Cage. The neutral ground at the center of the multiverse, where every door, every arch contains a portal to somewhere else, if you can just figure out the key. Every plane, every prime material, it's all connected through The Cage. The wrath of the gods and archfiends cannot touch Sigil, for they fear the Lady of Pain, making it the perfect haven to rest between adventures...if you can stomach the fact that all are welcome, and you might find a cornugon and a marilith at the bar with you, arguing with an Astral Deva about the moral implications of the actions of mortals.

This is not to say that Sigil is a safe place. So long as the gods stay out of it, and the city is not damaged, the Lady cares little for what goes on. A knife in the back will draw no attention from her, though it may draw the gaze of the Harmonium or the Mercykillers.

With access to all planes, and such a varied spread of 'people', Sigil could provide an amazing amount of possibilities across the level spectrum, all the way into Epic, and possibly beyond.