There would be two potential problems with that:
1- If you went over your last expansion's level, then your character would have to be locked. Otherwise, it would defeat the purpose of the xpac if your toon reaches the new xpac's level cap.
2- If they would lock the exp at one point from DING, like they're currently doing for the smaller trials, then the complaints would be that you can't do enough within your 30 days, as you're gated by the level lock.
How would you take on that sort of catch 22?