We all know that there is less incentive for players to remain at cap. You simply can not progress your character after you got all your destinies without tr'ing. People are also keen on making raid completions persistent through tr, so that they can go for 20th list while perma-tring. A lot is done for the perma-tr community, time to please the stay-at-cap community.
What about spending a tr-heart, and getting a buffer of the amount of xp which would be needed to get back to lvl 20 or epic from 20-28 or iconic 15-28, when you fill the buffer (through gaining xp on level cap) you get the past life. This is of course a little naive, more thought has to go into it. But the idea is clear. Progress your character at level cap!
I do not especially know if I even myself like the idea, but I think it is a great counterpoint to all that perma-tr pleasing and it is time to please the stay-at-cap community.