As much as their are tons of ways to earn xp, I too agree it would be nice that once you complete the main scenario, you can unlock a reset quests button, remove rewards, just have them give xp
To add, it's not just to get quick xp, sometimes it's nice to go back to a quest, a lot more wiser.
Yes I'm looking at you annoying quest in Coerthas against the guy and his wyvern, or the annoying sylph one to find klaxio