The HK don't feel that big of a threat right now, I dare say they look more like the underdogs than anything else. One of the things I can see Oda doing to hype them up is to have them take down a living legend like Gaban.
It'd be something similar to what happened to Garp. It'll raise the threat level of the HK while still preserving Gaban old legend status. That way everyone wins.
I don't know if the three current HK in Elbaph will be enough to accomplish that, but I guess the less HK needed to take him down, even underhandedly, would be plus for them threatwise.