Demon Hunters have brought a brought so many character customization options to elves that they are almost all new races. Of all the classes that could be added next (If blizzard feels like there is any room for anymore classes at all.) Tinker seems like it would bring the most uniqueness and still fit well within the game. I would love to see how the same principles of customization the Demon Hunter brought could be applied to Tinker, and by extension, gnomes and goblins.
Mekkatorque’s Battle Armor could be a hint of what a Tinker in the role of tank could look like. If we have a large battle suit we are wearing all the time, some customization options would make sense, but it’s also possible that a tinkers equipped armor would apply to your battle mech rather than your character.
Something I would really like to see would be prosthesis. Blizzard has some really cool looks that they have slipped in to other games and I see no reason to not keep that up in Warcraft.