I would love it if the experience I've earned on my main roughly 350 PA levels, would be the same experience across the board on my account. I will say that due to its current state I will never have it in me to level up an ALT that will be just as competitive. The task is just too daunting, but maybe that's just me.
I know there are a few games that have similar system to planar attunement and they are universal to the account. If the transition would be too difficult for Trion to accomplish, (taking all PA XP from all characters and combining them) I think people would be just as satisfied with the system being implemented and having their highest PA go universal across their account.
Naturally, to avoid any problems, the planar attunement xp your account has earned would not be available to your toon until you reach level 50. I think it would definitely encourage people to run alternative characters more and keep the general happiness of current players/vets in check.