Beyond All Reason – open-source RTS game built on top of the Recoil RTS Engine
github.com
github.com
Zero-K simplified the economy (metal cost = energy cost) but added a whole lot of mechanics like overdrive grids and made every unit unique. There is a lot more unit variety with mind-control, jumpjets, shields, unit-throwers, self-replicators, teleports, terraforming, etc.
Their design blog is also very interesting: https://zero-k.info/mediawiki/Cold_Takes
Perhaps still the only game whose loading screen dedicates time to loading explosions!