I love this idea!
There are other reasons one might want to replay a quests too, such as lore.
I've noticed quite a lot of players are really into lore, so they may want to replay so they can reread the dialogues and such.

Then there are also players like me. I came from a game where the story was...meh...click skip click skip. So, out of habit, when I started playing this one I wasn't really reading! It wasn't until I got to the Sylphs that I realized that, this might actually be worth reading! Unfortunately, short of starting a new character, there is no way for a player like myself to replay those quests so I can catch up on all the stuff I missed out on due to my bad previous mmo habits!

What i think might work is having a check mark in the Completed Quest Log. Once checked the quest is re-enabled, and a Repeat Quest icon shows up at that quest giver. If the quest is part of a chain, upon completion, the next quest(s) in the chain are also checkmarked for Repeatable. If a player no longer wishes to see the icon for that quest, it can be unchecked.

As far as rewards go, I would say either a pure exp-only across all quests, or they actually figure out what the repeatable rewards should be for each quest.