+20 LR or Free Past life + get back to max level for every single one of your characters... that sounds reasonable. Make them bound to account please on top of that.
Edit: After a bit of thinking, while the TR + stone still makes no sense to me, I'm starting to think that a complete revamp from scratch (LR) might be the way to go if faisible.
It's just there are some little problems still: You have an old Bard that you don't play anymore, now you can swap in to a full Wiz and TR instantly to get the PL. Or just end up with any toons you want if you had a couple old characters. I think that's still less hurting then just screwing some people's build.
The problem lies within the amount of free LR some people will be asking for. One per character is just... not needed AND too much.