I admit my idea is too hardcore, and even people who like it might get annoyed at some point, including myself.
kilgore's idea is better.
but maybe it could work for solo people too? kinda like Gargantua said, a new type of special encounter with a higher chance to spawn, you activate something and if you hold out for an X amount of time you get the reward. except the number of attackers has to be at most as the number of defenders.(although people could use alts to attack themselves and lock the instance)