From a Lore standpoint:
An Engineer builds as Hobby, or to improve on his/others life. (Profession)
For a Tinker, engineering IS his life, his very ambitions in Engineering is driving him forward.
Anyone cane be an Engineer, but not anyone can be a Tinker.
THis is something I would buy.
A class is the essence of a Character. While a Profession is just what he does. Again: Afaik Mekkadrill is considered a Tinker (Correct me if I´m wrong)
For the same reason I would also buy a kind of Mage who speciallizes in Enchanting. He Bolsters his allys with powerfull enchantments, and distorts enemies or their surroundings. Could also work, though this could possibly mix bad with the Existing Mage. (Note this is not a Recommendation for a Class)