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

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) https://en.wikipedia.org/wiki/0_A.D._(video_game)