(This is all in reference to PvE: mostly open world, metas, and fractals. Very new to PvP and WvW, so these suggestions don't address those game modes at all.)
I've been playing the game on and off for a year, and really like the mesmer class: it's unique class, has great visuals, and has a really nice toolkit.
However, whenever I play a spec that isn't virtuoso, I just feel completely hamstrung by clone generation. Having the clones disappear upon the death of the mob makes it feel clunky to play, especially in the open world and fractals. I really like to play support builds, and when I compare how the mesmer boon generation feels in comparison to something like the herald, it feels slow and kind of useless unless we're sitting on a boss, because I just cannot get clones up fast enough to do my job before the mob dies. It's a base profession mechanic that I think really isn't working that great across a pretty significant swath of PvE gameplay cases.
I think that allowing some "grace period" during which you can switch to a new target and not lose clones would make a world of difference. Let the clones have an expiration time, so we still need to worry about actively generating them, but don't have them vanish as soon as the one mob in the swarm we are fighting dies. I'm imagining your clones stay up, modulo some baked-in expiration time, as long as you are in combat and haven't shattered them. I know this is not a new idea, but I wanted to raise it again after being reminded this week of just how clunky mesmer feels outside of virtuoso. We can already lose clones to area damage, so that kind of clone management stays in place. We're motivated to shatter them, because boons and damage are very much tied to shatters (except maybe poor alac mirage). This kind of change would help with the flow of combat and the feel of the class.