I think it should be based on all breakables, both boxes and statues. Some bonuses could be:
- Increased chance of getting souls/buffs
- Soul/Buff value multiplier.
- Additional things in pool of bonuses, maybe boons?
Special objects may be a cool idea, but it's hard to imagine something fun and useful. If there were consumables/single-use items in the game, the boxes could have drop them.
I also can suggest the idea of triggering a sudden event. For example, something like a "gold rush" for you or team.