Official Blizzard, idk if you've played just the original basegame or the Brood War expansion. There is Starcraft Remastered, which has updated graphics and usability improvements but doesn't add much other than that over the original.

An open source Brood War engine seems to exist but development appears to have stalled.

For SC2 the coop missions and the custom games are imo the most fun to be had in that game.

In terms of RTS genre, BAR is an epic scale free and open source RTS that feels like a distant cousin, but doesn't have the deep lore that Starcraft has yet.

I'd also recently caught word from a fellow Lemming about a new game in development called Stormgate, worked on by some ex-SC/WC devs under a new studio. Gameplay has a very similar look to the former two.