XP currently serves very little purpose for the vast majority of the playerbase whose characters are capped. When Mod 9 is released, it's very likely my 8 current toons on Thelanis will all be capped within a month. What's the point of xp anyway? They might as well just bump us all to level 20 as soon as Mod 9 is released.
My suggestion is to make xp unlimited. After you reach cap the xp is accrued towards bonus action points. The cost of each bonus AP is increased. (Example: 0.5 million xp for the first AP, then 1.0 million, then 1.5 million, etc...). Combining increasing xp cost per ap with decreasing xp reward due to repetition of quests would make it exponentially harder to reach the next AP reward. (This would put limits on the benefits to extreme powergamers.) But at least there would always be some value to xp. There would also be more value in doing a diverse group of quests (once your xp for certain quests vastly decreases due to extreme repetition.) Also, there would be some value in just running the quests you like. For example, if you love Gianthold Tor to the extreme, then at least everytime you run that quest you would be getting some xp. (Good for fun quests no longer at cap where loot incentives are not a sufficient draw to bring capped characters into those quests.)