Those who played GW1 will remember a condition called Deep Wound. For those that did not play, it reduced your max HP by 20% and reduced healing by 20%. It was prevalent on martial classes like Warrior, Assassin, and Dervish but also Mesmer which is a caster class, and was a staple in both PvE and Competitive play.
I think it should be reintroduced into GW2, with both the HP reduction and the healing reduction. There are a few effects that reduce HP already, namely Lich Form 5, so the mechanics are already there to by used for bringing Deep Wound back. Having the healing reduction stack with Poison's healing reduction would also help versus bunkers in competitive play.
I'd propose letting Mesmer, Thief, and Warrior gain access to Deep Wound (this mirrors the availability from GW1).
For instance: Put Deep Wound on Cloak and Dagger and on Backstab for Thief.
For Warrior: Deep Wound on Savage Leap and Eviscerate. Maybe Even Rush.
For Mesmer: Deep Wound on Blurred Frenzy and Chaos Storm.
Any duration would have to be 2s-4s as it would be a powerful condition in competitive play. Being a condition it would be clearable. For boon conversion purposes I suppose it would convert to Regeneration, but if a new boon ever occurs it and Deep Wound should share a row on the table.