One idea is to visit a used bike shop or co-op and just take note of the fasteners on different bikes. You'll quickly identify which tools are ubiquitous, and which ones are more specialized. For instance 3 to 6 mm allen wrenches will be on virtually all bikes made in the past few decades.
Now think about the design of the tools as tools. For instance, larger diameter fasteners usually need more torque. And the tools need to be robust enough to withstand repeated use. Having the right metallurgy, and controlling your supply chain, will be important for the tools to be durable and precise.