The funny thing is that you don't even need to get rid of engineering. I would honestly give Tinkers a bonus to engineering and alchemy, and give alchemy and engineering professions recipes that specifically benefit the Tinker class.
For example, give engineers mods that effect the appearance of the Tinker claw packs and mechs. Alchemy could have potions that enhance the Tinker's resource/fuel. If done right, a Tinker class could actually create an interesting synergy between classes and professions.