A lot of people have discussed some alternatives to the Milestone advancement in the core rulebook. I can see why since it involves a lot of voting and people might want something a little more like the XP system seen in other RPGs. For those of you searching for an alternative, readers Michael Freeman and WhoIAm are here for you!
The crux of this system is using Episodes as a currency for advancement. When you’re part of an episode (whether that means a game session, a mission, or something else for your group) then that participation gets recorded. You then spend those episodes to advance your character. The options are the same as Milestone advancement but the different mechanism for rewarding advancements gives some more control to players and GMs alike. If this is appealing to you then you can thank Michael and WhoIAm for their good work!