Due to becoming aware of the no lv up cards after lv 300 thing, I propose a free lv up reset once you hit lv 300 that would send your lv back to 15. That way rebirthed players can't go into Crest. After rebirthing you would gain some sort of symbol showing that you rebirthed. For how exp would work since Im pretty sure people still want to beat up these rebirthed players for exp more then 15 and rebirthed players still want to gain exp as if they were a low lv. I propose that the system would keep track of your total lv, and would dish out exp to the enemy by the difference in total lv and their total lv if their current lv is higher then the rebirthed current lv, but when you win against a enemy, it would give you exp based on the difference of current lv and enemies current lv. Er I might have created some kinda weird error with that exp system, so please comment and put in suggestions on how to fix that.
Also in addition to this there should be a lv reset potion which acts like a rebirth, except you pay for it with gran. Not sure what the price should be on that.