When you reach the current level max, how about you still can earn and keep all the XP you manage to get?
It would show up as XP gained etc, but would only readjust when future levels increase.
If you currently are 100 you notice that your XP bar stops at whatever XP took you over the 100 and freezes there. Why not let the XP accumulate and when they do raise the levels, it would all go towards the new level caps.
If the current projected level for 101, is 110 million, and you reach it, you would not gain anything at that point, but you will when the caps increase.
You may gain 1/2 a new level or several levels. You can still do quests etc, while noting the XP is increasing and not being wasted. Of course all TP would still be as it currently is.
Hope this all makes sense. I am not the best explaining things.