I don't think anyone would argue with more branching narrative as long as it didn't compromise the overall story quality. So, while GW2 did, and your choices maybe even led to different fights and scenarios, they still eventually wound up in the same destination(s). Nonetheless, it feels like to me that all MMOs suffer from the, "You're the Hero(tm)!"-itis, where even if factions exist to facilitate some aspect of (likely terrible open world) PvP, we're more of a spectator of all the world's going-ons.
I'm actually kind of waiting for a game that lets us bring up multiple characters at once, serving as more a personal team or faction. This makes it a bit harder to get away with silent protagonists even if we may arguably have a "main" that defaults as the "leader" of the bunch. This, however, would also demand a solid NPC AI or Gambit-like system, but if the player so wished, they could play anyone in their unit and might even get split up for specific tasks/missions. Given the prevalence of the trinity (and I feel failed attempts to "abolish" it from some games), this could also go a long way in helping people teach the various roles even if they may stick to their preference in the long term. PCs could still mix-and-match groups, though, and "big conflicts" could technically be even bigger if you consider one person is literally bringing a party. I'd also say there's a greater economic benefit to this approach of play, where instead of needing to gear up a single character, your whole entourage needs attention. Some might argue that applies to alts in more the current mold, but it seems more like to me people still wind up with a main pick and skimp on their secondary characters in some manner (even if it's just relying solely on quest rewards to progress). If keeping your "team" up to speed is more imperative, I'd say economies will remain stimulated for longer periods and this would still carry over to those who still braved alts with their own teams. Then again, the fourth wall could be broken a bit the ability to make "main" characters on the account party-able.
More cynically, MMO stories are fleeting. Were travel and fights not required, we could probably press enter at a reasonable pace for a lot of expansion sets faster than an hour of TV programming, and that's not a problem I see going away anytime soon since dev manpower is finite. This is where I instead default to giving players the means to create content for themselves and others. Some would tackle this solo, others would form teams and delegating tasks like the story writing, building locations, boss encounters, etc.. Most will probably be mediocre or below average, but others would be good and maybe even adopted into the core game to help broaden the depth of the world. Things like this are also why I stress we should look to these games evolving titles instead of just, "Raid for a few months if you can, take a break, repeat."