Eternal warfare is, in my opinion, a criminally under-rated game, It is a turn based strategy game with many unit and building types, it has online multiplayer support, a skirmish mode for vs A.I., but the campaign is currently being devoloped (I think, I don't know if they are making the game, as the last update was 113 or so days ago). There are never any servers and it would be great to just see the server browser have at least 1 server that's active. I would suggest anyone who has played Advance Wars, or other Turn Based games to give this a try! (Honestly I would suggest this to anyone, seriously, give it a try.)
https://spacecpp.itch.io/eternal-warfare
(Edit: The creator is actually active, i'm just not sure if he is still working on this game)