I posted this on another thread but thought that it deserves its own suggestion thread. I first read about this idea from someone else's post and decided to elaborate on it.
The problem with the bug as it is, is that having only hit stun in effect without hit invulnerability allows any weapon with an attack rate faster than the hit stun duration to lock an opponent in place while dealing damage. Prior to the bug, this was prevented by having a hit invulnerability period that lasted for the same duration as the hit stun (though skilled players could still position their follow up attacks to land right after the effect wears off). However, this mechanism prevented weapons such as the blitz needle and cutter from being effective as most of the hits pass right through the invulnerability frames. Now since we've already learned that having only hit stun causes many weapons to be broken, the only reasonable way to allow these rapidfire weapons their full damage capability is to remove both the hit stun and hit invulnerability effects for those weapons.
The idea is to allow multihit type weapons like the cutter, needle, and antigua lines the ability to do continuous damage with both hit invulnerability and hitstun disabled. That way they can be used for high risk/high reward DPS. Since these weapons would no longer be able to stun lock their victims, successful users would require situational awareness as to when it's safe to attack (such as against distracted opponents).
Guardian shields however, would have to still experience the same interval of hit invulnerability against these weapons to prevent other classes from having too easy a time depleting their shields from distance.
It may also be necessary to reduce the damage of the needle guns, particularly the charge attacks. It takes about 3-4 hits from the charge to kill someone, which given the high rate of fire, doesn't allow the victim enough time to get out of the way after taking initial damage.
All weapons that are three attacks or less would revert to the original mechanics in order to limit their DPS, and reward good timing. Weapons like the flourish, brandish and warhammer do not rely on rapidfire to accumulate damage and were fine under the old mechanics.
TL,DR: Make weapons have either both hitstun and hit invulnerability or neither, based on their attack rate. Prevents buffing of already good weapons, allows new weapons (cutter, needle) to be effective without being OP.
EDIT: Added preamble in italics
Well-said.