I think it would be really cool if an Assassin wore a mask constantly, to hide his face due a disfigurement he suffered when at a younger age. He'd only take it off when he needed to eat, drink or when he knew he was alone. One of the facets of customization would be the ability to change his masks, though all of them would be face-obscuring. I'm a sucker for obscurity.
Now, I'd like to ask you another question. What would be the signature weapon of this Assassin? Every Assassin's Creed game seems to have a signature weapon for the game's protagonist, like Ezio's double Hidden Blades in AC2 or Conner's tomahawk and pivot blade in AC3. I had the idea of a Hidden Blade that used clockwork to extend the blade to multiple levels, capable of acting like a normal Hidden Blade or a forearm-mounted short-sword... but that was kind of stupid. I don't know, you tell me.
But if there is one weapon I'd like to see, it'd be the crossbow. It's powerful, more accurate and easier to use. Just imagine upgrading it to give it a retractable bayonet akin to the Hidden Blade, add serrated edges to the limbs or even add a primitive scope for long-distance shots... and perhaps one of the final upgrades would bump up the crossbow to a repeater, making it some sort of medieval machine gun. Use different types of bolts like flaming bolts or poison, if you want to make sure the target dies.