Well, I was talking to Boswick a while ago and he said they're going to be adding more relevant content to each tier as well as trying to introduce new procedurally generated stuff as well. They're trying to find a good balance with content right now so it might be a while yet before they get around to ordering the content they have, but if there are any particular levels you think should appear more frequently higher up, maybe it would be a good idea to make a thread about it in the General Discussion.

There are, for example, a number of levels I don't think should appear higher up, like the Totem Trouble levels or Emerald Axis ones.
It seems to be a waste of their design to apply pressure even to well geared players, when everyone in Tier 1 is equal regardless of the gear they're wearing.