What bugs me the most is that we don't have more low tech tools other than the current primitive ones. What I would like to see is tools that are made of obsidian and copper, that way we got something other than Iron tools right after primitive.
If these two new lines of tools were added I would structure it like this: Primitive -> Obsidian -> Copper -> Iron -> Steel. Obsidian is a stretch since it is only found near volcano's. On the other hand, copper would really help with the gap between primitive and iron tools, perfect for newbie blacksmiths.