I always thought about replacing HK's passive with one that strengthens their buffs, just like guard's passive strengthens their heals. But in another thread about rebalancing someone said that this could be op because HK's buffs are already pretty strong, so I began to doubt this idea of mine.
So while I do think that HK's passive in its current state is pretty useless, I can't think of an alternative that wouldn't be too powerful tbh. Especially since HK is currently not in need of any buff. It would help to see some changes on the guardian like the ones Kali suggested and the cross fixed, then maybe based on that situation the HK passive could be replaced by one that fits the new balancing.