Here's the description of Melmetal that was posted on Serebii:
"Melmetal was worshiped in ancient times as a Pokémon with the power to create metal. Because of its Evolution from Meltan, its body has hardened and is now incredibly sturdy. Melmetal is made of liquid metal, and its flexible arms can be whipped forward to deliver a devastating punch. It can also execute a powerful, unique Steel-type physical move known as Double Iron Bash. By using the hex nut in its chest like an axle to spin its arms around, Melmetal can deal a double strike to its opponent. The weight of the heavy hex nuts on its arms combined with the spinning motion gives the Double Iron Bash move extraordinary power. "
Given what we have here, I can imagine this Pokemon having a decently-high Attack stat and some impressive physical Defense. As for possible abilities in the future if it doesn't get a signature ability, I predict Sturdy (I know just using the word "sturdy" in a description doesn't automatically point to the Sturdy ability, but it kinda makes sense for this mon), Clear Body/Full Metal Body, or maybe even something out there like Liquid Ooze or Gooey.