Button mashing is the obvious choice for a BAD answer. I can almost personally PROMISE you it will go like this:
Two guys get into a melee struggle. It will work pretty much like a power struggle does, witht he meter and using ki to pump out attacks. The difference is it will have the anmimations of them "flash fighting" (that's what that fast melee brawl-style fighting is called) until eventually one wins and is slammed with a cool-looking finisher. Likely the damage this hit does will be a ratio based off of how much ki the two players put in, because A.) power struggles and block struggles are already like this, and B.) it would be dumb to bust your ass just for a regular damage melee hit. Either that, or it will send them flying much further so as to guarantee they hit a wall or something.
Assuming they do it this way, I think it would be cool if you took constant minor damage the whole time you were in the struggle (VERY minor, like in increments of 1 or 2), and had the option to teleport out of the fight at the expense of a huge chunk of your ki (that way it's not like you can struggle, and pull out at the last second if your going to lose, because you already used up too much ki to do it in the struggle). Unfortunately while this is a great idea, too many pussies play ESF, and will just constantly teleport out of every fight, so I doubt that feature would even be considered.
Regardless, though, I'll bet my left leg that's how melee struggling will be implemented.
Don't suggest button-mashing. It's retarded. It doesn't add "Skill" or "timing" to the game. Not everything is supposed to take skill and timing in a game and this is just too straightforward of a thing to implement a whole retarded new system in when we already have a system included for struggling of varied degrees.
And if this team has a clue about design (which they obviously do from this incredible work they've done balancing things out and coding this bad boy), they'll never add a button mashing suggestion anyways. Button mashing is best left to consoles; keyboards and mice aren't meant to be mashed. Sorry.