[triilman]

I think this is just stupid question, but I wonder in FOSS there is no game project that created from active community for alternative of another game. When we face a problem like paid software or problematic software that steal our data, the community will make the alterative for us but it is less possible for game generally.

[card_zero]

I wonder what your concept of "game" is. Maybe your question excludes offline and 2D games. (Lots of roguelikes.)

In regard of "alternative to an existing game", there's 0AD, which resembles the Age of Empires series. https://en.wikipedia.org/wiki/0_A.D._(video_game)