I think that Léodagan needs a buff as does Arthur. I was thinking their abilities could be moved around a bit and changed as such.
Léodagan would make it so that you can use any building (including tavern and garrison) 2x per turn. This could be any building or any 1 building if using multiple buildings 2x in one turn is too powerful.
Since Léodagan would now take over Arthur's tavern ability, Arthur would get a new, different ability. I like the idea of Arthur sort of "fighting along with his knights" and being the deciding factor in a battle or ordeal. So my idea was that when you have Arthur, he gives you the option to complete any mission that you have all but one of the needed dice placed on the card for already. So say you have 3 (2)s on the Gryphon, you can use Arthur to complete the card with a 4th "Arthur" dice. Using him this way would remove him as leader (he's now "in the field") and the (3) dice in the leader or tower spot would either go to the round table; go to the tavern; or maybe even is destroyed like when the black knight ability activates if the ability needs to be a little weaker.
I think these would be neat abilities cause this way you have Arthur for missions, Léodagan for buildings, and Lancelot for Quests.
Also, a different ability for Accolon if that other idea doesn't fit is that he could alternately block you from doing one quest or the other each turn. So like, blocks placing dice on the Graal on the first turn, then blocks the other quest on the 2nd turn, back and forth like that.