Reducing bullet damage is a very interesting idea that would fit well with Harbor's water theme, but imo it doesn't really help his relevance as a controller. Your idea (buffing the slow and removing it for allies) is better because it buffs his general viability, and although the Anderzz idea would help him on maps like Icebox where he's chosen purely for his retake utility (cove), it doesn't actually improve his overall utility as a main smokes agent