It just occured to me that I am (as Japan) the tech-leader in my world. I don't have access to either Oil nor Uranium. Sure, I know where I can get both, but if I choose not to take control of some Oil/Uranium source, I am still able to build
Airports without ever having seen Oil. And down that path is steampowered Spaceflight.
As I see it, that shouldn't be allowed. So the rule would be:
Nuclear Power is not possible to research without either
- access to Uranium
- or Open Borders with another nation that has researched this technology already
Same goes for Aviation, cannot be researched without
- access to Oil
- or Open Borders with another nation that... see above.
I don't see any other techs that would need such a rigid resource restriction, except possibly Steel (requires Iron) and Steam Engine (requires Coal): but I think those two are easy to handwave, as the raw materials are possible to get somehow.
Flight by itself can similarly be handwaved (some coal-powered steampunk engine, or run the plane on salad oil), as can Fission (small amounts of nuclear material are available somehow).
But to develop the Aviation industry or to go for Nuclear Power: you'd need either access to large quantities of the resources; or you get the inspiration from a friend.
Trading for the tech, and espionage are also both viable, my issue is only with initial research when you have no access to the fundamentals.