Other games offered by Microsoft for free on X0 are the first crackdown. It comes from Xbox360 but is also compatible with Xbox One and with 4K support for Xbox One X. Unlike PUBG and PES 2019, which are free for weekends, it looks like this game will still be free for players too.
Microsoft hasn't announced an official announcement, it seems everything will be heard tomorrow at X0 event.
Crackdown can be activated for free at the Xbox360 store.