The issue with MMORPG avatars though is the sheer amount of customization available to them. Take the Pact Commander from Guild Wars 2 for example, they can be a Sylvari, Charr, Human, Norn or Asura (pictured left to right):
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Only the Sylvari and the Human would be able to be alternate costumes of each other, as the difference between Norn and Humans is that Norn are bigger, but big, Asura are tiny, and Charr don't even have the same body shape.
Then there's the fighting style. Before we even get into Guild Wars 2's combat mechanics, we have to decide what class we're depicting. Of which, there are 9: Elementalist, Necromancer, Mesmer, Ranger, Theif, Engineer, Warrior, Guardian, and Revenant. None of them have any strong parallels between each other aside from their armour class and the weapons they wield. There's just no way you can fit an elemental mage, a mage that builds corpse minions, an illusionist, an animal handler, a flippy dippy assassin, a fighter who protects allies with magic, a no frills soldier, and a soldier that calls upon the power of famous dead people in a single moveset, nor would it be feasible to add all of them. There also isn't one that is any more important than the other so adding any of them by themselves would be wierd.
All MMORPGs have this problem to some extent, though perhaps World of Warcraft could get away with adding a bunch of basically human races like elves and whatnot, but they'd all have the issue of what class to pick. Unless there's an infamous one that everyone loves, I don't see the player character of any MMORPGs getting in Super Smash Bros. like, ever. It'd be much more likely to get an NPC like Rytlok (Guild Wars 2), The Litch King (World of Warcraft), or Y'shtola (Final Fantasy XVI: A Realm Reborn)