I would like to see interrupt builds become a thing again. Secondary to that, I want the old behavior of Mantra of Distraction restored, where you could charge it if you were out of ammo, and when you completed the charge, you would reduce the cooldown of Diversion by 15 or so seconds.
Interrupt builds were fun, and felt really good to pull off, particularly with the obnoxious amount of stability and aegis in the meta. Coming back to GW2, I was disappointed to see that a lot of the talents, traits, and skills that made mesmer interrupt possible have been changed for the worse. They weren't even meta builds when I started my extended break, so I'm not sure why such drastic changes would have been undertaken to nerf them.
I'd like to see mesmers get a real stealth or a real clone build. I mean, sure we have clones, which are essentially just vehicles for shatters. I'd like to see mesmers get tools that enable them to trick opponents into attacking clones, and then punish opponents for doing so. Something to make the class feel as tricky as the lore makes it out to be. As for stealth, we have tools for it, and they're great. I would like to see us get more.
Mesmers are pretty fun still. Though I will admit to being disheartened upon first reading about the Virtuoso not having clones. It felt like anet was giving up on trying to make the core concept of the class work. It would be like releasing a thief without initiative, a ranger without pets, etc.