The first question to ask is "What role can a melee hero serve in the game?" I'm not quite sure what you mean by "assault class". A DPS hero? A flanker?

The problem with a melee here is that outside specific range their damage instantly disappears and since they have to fight up close they are extremely vulnerable. This makes them very situational and easy to punish. Since Winston's weapon has melee range and damage which cuts off suddenly like this, he can be considered to function as a melee character for the purposes of this analysis. Reinhardt is the other melee character in the game. Both fit the logical roles for melee heroes. Winston is the closest thing to an assault melee character.

Every character forced to fight up close has an escape and/or invincibility ability to deal with the risks of jumping in the face of an enemy team. This is why Symmetra was given a barrier she can project. Reaper can wraith, Sombra teleports, Mei freezes herself and builds walls. Rein and Winston have barriers. A melee hero needs something like this.

A melee hero is hard to design as DPS, since high mobility abilities are common in Overwatch heroes, allowing them to stay out of your effective range making you do zero damage. You can give the melee hero a lot of mobility to make up for it, but then getting in the range for high DPS would be easy, and feel lame to play against. For the high mobility melee hero you'd have to make the DPS pretty low. Then health needs to be made high to make them still useful... and we have Winston.

The devs actually intended Genji to be a low health DPS melee hero with high mobility, but he was impossible for them to balance so the make his sword fighting style into his current ultimate.

Alternatively, you can give a melee hero a reliable way to negate taking damage when an enemy is outside their range, evening the playing field. This ability shouldn't make approaching enemies mindless, or DPS needs to be really low, so reduce movement speed while it's used so enemies have a chance to back up. This is Reinhardt... It's kind of hard to come up with an alternate way to negate damage. Unfortunate an ability this strong centralizes the character as a tank.

Let's try something totally different, a no mobility, no barrier melee hero. He'll need high

So what role would your hero fill that these characters don't? Your boomerang is similar to fire strike, but worse.