Using an aura barely makes any difference, atleast I didn't notice any difference when I tried runic accuracy at Zilyana.
Proposing the following changes:

Vampyrism: Heal by 5% of damage dealt. -> This one is fine. Great for casual PvM and bossing.
Penance: Increase defence by 10%. -> Make it decrease damage taken by maybe 20%? I.E Instead of taking 30 damage it would be decreased to 24. Would make it a great choice when it comes to solo bossing
Sharpshooter: Increase range accuracy and strength by 5%. -> Make it increase only accuracy by 15%. 5% accuracy and strength bonus barely makes a difference if none at all.
Runic accuracy: Increase magic accuracy and strength by 5%. -> Same as above.
Brawler: Increase melee accuracy and strength by 5%. -> Make it increase both melee accuracy and strength by 15% but decreases defence by 10%. Would be a great aura to choose for group killing Godwars dungeon bosses with melee. Melee is a bad choice and rarely used when bossing solo. Also would be a great choice for slayer

Poison purge: Poison damage is changed into healing. -> No changes needed I guess. Great choice for kalphites and KQ.

I agree with everything except your suggestion on Brawler. I don't think auras should have downsides, the better way to change it in my opinion is by giving it a buff (10% increased strength and attack accuracy).

I agree with everything except your suggestion on Brawler. I don't think auras should have downsides, the better way to change it in my opinion is by giving it a buff (10% increased strength and attack accuracy).

Then again the 5% buff wouldn't be enough in my opinion, 15% seems like a sweet spot to me. Negative defence bonus is to compensate with the power it gives as it increases both accuracy and strength. It's a melee aura anyway which means you'll have a lot of defence compared to mage or range gear.

I agree with everything except your suggestion on Brawler. I don't think auras should have downsides, the better way to change it in my opinion is by giving it a buff (10% increased strength and attack accuracy).

Bosses already hit with nearly 100% accuracy already, it's barely a downside.