For another oss rts engine that already runs multiple online games, there's springrts. It was originally intended for Total Annihilation so it tends towards TA

For another oss rts engine that already runs multiple online games, there's springrts. It was originally intended for Total Annihilation so it tends towards TA-ish play, but many developers take it in exciting directions with lua. That said, spring grew very organically and it shows. A fresh start like this probably has a lot of advantages.