This idea is simple... basically make it so only land factories can build engineers. This means Air and Naval factories cannot.
This is a small change, but it would be effective in a couple of ways. First, it would be a nerf to air first strategies without directly nerfing any air units... second, it makes sense because engineers are land units. Third, if we made the change to quantum gates that has been thrown around... allowing them to build SCUs and any other (non XP) land units... it would be necessary to ensure that somebody did not tech air or naval and then gain access to land tech without first paying the required 4000 mass to be able to use tech 3 land.
Thoughts?