Honestly I don't think a max ranged potency gain would work too well, most end game content is designed around stacking as a party on the bosses butt until mechanics require otherwise. Most fights the group moves as a collective. Something like this would warrant an unhealthy gameplay style that would punish healers and I doubt it would do enough potency to warrant the "healers adjust" blm mentality. I think that walking casts are a step in a more progressive direction but adding max range potencys would lead to the old "This fight is unfriendly for mch" which has plagued melees and other roles in the past, thus leading to some tiers being deemed role friendly or not. It's not a bad idea, but I don't think it'd work well in a pve enviroment especially on a savage and ultimate level, it could also lead to more casual players that are healers feeling stressed about dealing with these players. If a system like this were to exist however, I do think it would be better suited to a placeable turret, where positioning it far out but not out of range, could be rewarding.