There were a number of things in GW2's design that worked against it for having fun and engaging dungeons and raids. The lack of a properly defined role system was a major aspect of that. In order for a mmo boss fight to function properly and be fun and challenging, players must assume distinct roles necessary to defeat boss which for the most part boils down to players adopting the roles found in the trinity plus any boss mechanic extra stuff. If roles are not enforced by things such as class, job, etc. and everybody can try to do whatever role they want, battles turn into an uncoordinated mess and it is very difficult to design and tune such content when how the players will engage it is so unpredictable. While you can successfully create some variance to the classical Trinity setup such as allowing stance changes, primary+secondary roles, giving different roles/classes extra utility, in the end using the trinity system or some close variant of it is best for this kind of game and content.