The whole modern military theme wasn't in the original plans for the first game. It actually started out as a fantasy RTS, until they believed that fantasy was niche and modern military was more relatable with the then-recent Gulf War. The direction of what would become an entire series basically hinged on this change in direction.
Command & Conquer: Continuum was going to be a Command & Conquer MMORPG.
There were several non-starter games with the "3" in the title. While we would ultimately get versions of both, between Generals and Tiberium Wars there was a developed version of Red Alert 3 that was ultimately cancelled, and Westwood's C&C3 (and its would-be first 3D RTS produced in house) went down with Westwood and much of its development assets would lay the foundation for what became Generals.