Simple fix: Change cutscene skip to a vote which has to be unanimous to do it. Worked fine in Guild Wars. Nobody ever complained about the cutscenes as far as I saw, and I played Guild Wars a lot.

Simple, yet flawed fix: Boss battles can't initiate until everyone is inside the circle. Would work like the simple fix but now everyone would know who's still watching the cutscene. Complaining and votekick abuse would ensue.

Difficult fix: Less cutscenes in dungeons or splilt dungeons to story- and non-story modes.

Impossible: Somehow design party content in a way that patient, friendly and sociable players are the ones who most likely reach the end game.