We have
Insidious Cunning (+10%)
Devious (+7%) and
Discreet (+5%)
for being stealthy / successfully zergy and not killing many monsters to achieve the quest objectives.
Why not have similar bonuses for the - to my mind usually much harder - task of not breaking many breakables.
Given kills goes up to +25% for conquest, and breakables only goes up to Ransack at +15%, maybe limit the potential bonus for not breaking breakables to +8 - +10%.
Say,
Fastidious (+10%) - 0 breakables broken as with insidious cunning for kills.
Tidy (+8%)
Careful (+5%)
I'm not going to suggest percentage of breakables for the other thresholds, but ripping off something like those used for devious and discreet would probably be about right, and, as with devious, would probably seldom happen spontaneously without effort on behalf of the players.
I appreciate some comments will likely address that, "power-levellers don't need more free xp", and I do see that side to it, but I think it's a downside that doesn't hurt too badly in exchange for a genuinely fun upside. Also, I suspect it would be rare that zerg parties would achieve a breakable-free run (quest dependent) due to monsters thrashin' about when invisible things run past them at light speed (that said, they should achieve some bonus for a completed quest with minimal breakables broken).
I know I'd like to have the option of achieving a breakables bonus through skillfully avoiding them (careful with that greataxe / those fireballs), rather than the time-sink of an extra few minutes laboriously smashing sarcophagi like a House Jorasco graverobber. I'm all for the continued existence of Ransack, both the guild and the bonus, since seeing / finding every breakable is a difficult (I'm looking at you Chains of Flame wall-colour urns) or at least time-consuming (Free Delera) task in some instances.