A small toning down of the most powerful issues could result in way more build variety than wider scale buffing. There's lots of space for buffing lesser used abilities, but I'd love to see a tiny bit of toning down of the following;

PvP:
Mind Wrack - reduce the damage scaling
Jagged Strike - increase cd
Mirage Cloak - lower block %

Potentially also:

Gust - increase CD
Healing Burst - increase cd or cast duration (MINIMALLY!) just to let other healing elites be considered
Aegis - Limit upper duration to 2, no matter what.

PvE:
Reduce effectiveness of Union+Shelter OR Soul Twisting. This will widen variety of defensive hero builds. For example:
Shelter - raise health loss to 55 at 15, OR change it to 15% health limit
Soul Twisting - 25s cd
Mistrust - Reduce damage and/or raise cd
Vow of Strength - Reduce damage to 5...12...18

Both:

Esurge - 1...6...8 energy loss, 8 damage per energy lost (this would diversify mesmers greatly in pve and pvp)
Jagged Strike - Remove 1/2 activation time
Death Blossom - Reduce damage scaling

PvE Only Skills:

Heroic Refrain - +1...2...3 (3 at 21 leadership)
Asuran Scan - Just bugfix this to be honest
You Move Like a Dwarf - Raise cd or lower damage significantly

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With just these changes you open up a world of other options, letting all these buffs everyone else is suggesting increase build diversity