A pretty good looking RTS Battle of Empires : 1914-1918 looks like it's heading to Linux & SteamOS if SteamDB updates are to be believed:
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launch/4/description: Play Battle of Empires: 1914-1918
Looks quite a bit like Company of Heroes 2, but in a WWI setting.
It's a shame that a bunch of users are review bombing it right now due to DLC additions, but it does have plenty more positive reviews than negative.
Remember, never buy a game until the Linux version is actually available.
About the game (Official)
Battle of Empires :1914-1918 is a story driven RTS, that re-lives some of the bloodiest battles of “The Great War”. The missions include some of the most fearsome weaponry employed of the period. Sabotage, stealth, trench warfare, gas attacks, artillery and mobile infantry - BoE has it all!