Something they could attempt, let's say the level cap goes to 55.

From 40-55 new gear would be introduced that's tied into the new expansion, let's say something like "aetherial immunity" gear is needed which helps reduce the dmg taken from devestating attacks in that zone and without the aetherial immunity you'll take to much dmg to survive most of the AOE attacks in that area.

So while players of lv 50 il175 might be stronger and hit harder which might help them with some of the start of the expansion content, later on they'll have to obtain the new i60-90 gear that has the special attribute of "Aetherial immunity"

Basically with this concept they can reduce the risk of power creeping by having gear for new expansions be tied into the content in question without having to make creatures that need ilevel200 to beat.