According to Ubisoft, Far Cry Primal delivered stronger than expected sales and set a record for a game released in the month of February.
http://www.polygon.com/2016/5/12/116...es-performance
If they do offer a Far Cry game for free I think the first or second game in the series would be more likely.
Edit: Far Cry 3: Blood Dragon is the next free game.
http://www.pcinvasion.com/far-cry-bl...ree-this-month