I would love to see mercies have a specific use again, so they don't look like a failed Janus clone. What if they do something similar to the absolver, and do large damage to shields?
I can see usecases not just on firebases, but also against those units that have shields themselves, like the harbs, obsidians, and titans, (where strong shield DOT could be really interesting). This gives you a cool utility unit that remains relevant up to and including the T3 stage.