As a Hunter main, I'd love to have this but only if it's conditional, otherwise it would be OP especially on bottom tree. In line with the general Hunter fantasy, I would only make it an OHK on crit if the target doesn't see me. Since we don't have invisibility on that subclass, that would have to mean a sneak attack from behind, the side, or above. Should give a means to counter in 1v1 by just facing the hunter, and play into the hunter/prey fantasy.
Other options might be to have it charge like a handheld supernova on top-tree, narrow the spread significantly on middle-tree, or even have the three knives thrown in sequence when charged, also on middle tree - both would require that all three have to hit to secure a kill, say 64/knife or so.
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Changing the middle tree knife fan from a spread to a straight line by charging the throw... Now this is something I can agree with. The benefit of range is offset by the fact that all three knives have to hit, yet anyone who is standing still or rushes you in a straight line is in for a rude surprise. There is still the issue of how quickly hunter knives regenerate, particularly with ophidian aspects, but its definitely worth thinking about. Nice thinking out of the box.