You know, if the plan is to do the standard 2 and 3e thing of arbitrarily capping the whole works at twenty, then just shave those last five levels off and make the cut off point fifteen. It limits power creep and you can get most of the good stuff in the game that way. For people who want lower power games, this can work in their favor too.
Also, later expansion wouldn't be broken into generic 'epic' categories at that point, which (for some reason) turns a lot of people off. Esp. after the 4e implementation, if Wizards is going to do something similar to the epic tier, I'd like to see it supported. (Really, epic is NEVER supported. Aside from the companion/master series, it's never been supported.)
But for the core game, top it off at 15.