In short, a melee-focused item that allows you to wind-up a Heavy Melee slightly longer for extra damage (and range?). This can be used to bait out parries or just a general cool punching gimmick. While charging a melee attack, you could also give resistances akin to Apollo's Flawless Advance if you feel that charging up a punch leaves you too much like a sitting duck.
The damage increase could just be a "hold charge for more damage" akin to Smash Brothers' smash attacks (with maybe a meter visible to you, fig. 1).

Alternatively, instead of having a linearly scaling meter, the charge-up could be limited to a set amount of stages similar to how the Fire Emblem characters charge up their neutral specials in Smash Brothers, or how Tekken's Jack can wind-up Gigaton Punch up to 5 times for more damage. (fig. 2)

I'm not much of a balancing man so I'm not gonna throw out random numbers to figure out how this would be balanced, but I think it would bring some nuance to melee combat and add amusing scenarios where someone could go behind a corner and charge up a punch for a few seconds.
The damage increase could just be a "hold charge for more damage" akin to Smash Brothers' smash attacks (with maybe a meter visible to you, fig. 1).

Alternatively, instead of having a linearly scaling meter, the charge-up could be limited to a set amount of stages similar to how the Fire Emblem characters charge up their neutral specials in Smash Brothers, or how Tekken's Jack can wind-up Gigaton Punch up to 5 times for more damage. (fig. 2)

I'm not much of a balancing man so I'm not gonna throw out random numbers to figure out how this would be balanced, but I think it would bring some nuance to melee combat and add amusing scenarios where someone could go behind a corner and charge up a punch for a few seconds.