BAR (https://www.beyondallreason.info/) is an open source TA clone that proposes massive scale and that is totally open. After the recent disappointment over a series of failures to bring a new big RTS game in the last 2 years, the RTS community talks a lot about this one.

The mechanics are old school, as with basically all RTS, but the openness allows for far more experiments than one would assume in a proprietary game.