How about making the mesmer into some kind of spell mirror/powerhouse.
Right the Mesmer has a spell in Inspiration magic called Mirror or Reflection. Costs 25e 3sec casting and 60 second recharge (feel free to tweak and tune according to your own experiences) Enchantement spell lasts for 20 seconds and ends with the next spell that hits the Mesmer
What this does is whichever spell directed at the Mesmer, is reflected and affects everyone in their aggro circle. It has to be a spell that isnt AoE, but directed at the Mesmer.
So if its a curse, the curse is reflected and is instead targetted to all enemy units in the mesmers aggro circle.
If its a spell (that ISNT AoE) the spell is reflected and hit every enemy unit in the Mesmer Aggro circle. this works for both friendly and enemy fire, so the enchantement must make the mesmer a target for friendly foes (and as the spell isnt going to affect the mesmer its ok

If its a healing spell that isnt AoE or a maintained enchantement, the spell doesnt affect Mesmer and instead affects all friendly units in the Aggro circle.
What do you think?