I like the idea of increasing mana consumption. One thing they could add with that is a DPS ability that costs no mana but does little damage (kinda like how wands work in WOW). That way even if you're oom you can still do something. Then you could have the sweaty parsers do like full burst and burn through all their mana to then regen it for the next burst window.

Would add a ton of options for how much a healer is able to DPS and still be able to heal when needed (which would vary depending on how good the team is)

This is additional on top of other suggestions some had for DPS rotation.