I would love to see a system that utilizes the XP still gained after hitting cap. I'm still running lots of quests where I see 10-15k XP as the reward and was thinking perhaps one way to continue to reward players for all the work they do beyond 20 would be to give them an extra action point for every X number of XP earned beyond level 20. Sure, it could even be a ridiculous amount, let's say 500k XP to gain 1 AP for a maximum of 5 extra AP earned beyond level 20 or some such but at least it would provide a reward system for those folks that don't want to TR. TR's would benefit too once they cap and decide not to TR again so they are still incrementally more powerful than a non-TR would be.
I think this fits within the boundaries of building and maturing a character. The character is going to continue to learn more/become better over time so a gained AP or two makes sense.