We are planning a similar system.
EXP (official title pending) can be earned by playing games and activities passively and by getting achievements. Each game has its own EXP meter. You could have 6,000 EXP for Minigolf, thus earning the badge of Tour Pro, but only 50 EXP for Virus (Zombie Bait). As you earn EXP, every so often you earn a milestone (3 or so milestones each Game World). The badges would be viewable on your in-game profile and on the scoreboard. We are thinking about a Legendary store that unlocks when you reach a certain EXP, but it hasn’t been fully discussed (and there are flaws with this store idea).