I don't think this is really necessary. I mean, the challenge log is just another bonus to complete content you already have. I agree there should be some rewards in place so XP isn't wasted on max level classes, but that's the only change I would wish for. It's already incredibly easy to cap Esoterics with roulette and PVP and Alexander runs. So, what would happen is, I would cap out in a day or two and then lament the wasted Esoterics from roulette on the days thereafter.