Interconnecting the servers rather than merge them in that AH could be a single database serving all of the servers. game connection between servers could join the players into a single group for questing.
if the quest them selves were on a dedicated server were players would transfer to that server to play the quest and then community back to the home server while leaving. still being connected through the Group channel for communication as they are still grouped.... it would be a massive undertaking to redesign the entire game.