Okay, so I have the buckler, but i think it could use some additions. its base use is okay, but not many people really care for it, since the speed increase is really not that necessary all the time. (and frankly i don't see a noticeable difference between my Avenger with/without it)
But this is what I was thinking: the stats stay the same to offset the bonuses, but each upgraded changes which bonus you have. So it basically has different upgrade trees/tiers, whatever.
One could be Combos (add one strike to weapon combo [Avenger's two-strike becomes three], upgrade of that is two, and so on.), another could be Movement speed (small increase at first, then higher levels as you upgrade it), and one could be Range (gun fire normally travels 3 blocks, now travels 4 and so on).
Just something simple like that to make it more interesting and worthwhile. Each one being a different color for distinction (Combo = Red, Movement = Green, Range = Blue)
Anyway, that's my thought.
The combo idea is too off-base a mechanic for a single piece of equipment, making it the ultimate DPS enhancer for the heavy blades. If they were planning on adding it, it would have to come with sweeping weapon changes.... I'd mark it as a large "maybe" project. Range is bad though... PVP nightmares.
Rather then tack on different abilities at each setep, why not rework the entire concept. This would be a good opportunity to use multiple bonuses and using the lowered defense theme to offset it. I tired working out some examples while keeping to the +3 total, but couldn't find a combination that stayed relatively balanced.
Swiftstrike Buckler(3*/4*): ASI+2, CTR+1
5* lines:
Rifleman's Guard: ASI+1, Gun Dmg+2
Swash Buckler: ASI+1, Swd CTR+1, Swd Dmg+1
Blast Shield: CTR+1, Bomb Dmg+2