Microsoft has announced plans for an Xbox Live rewards program based around players' achievements.
We've hit 50,000 followers, which means it's time for an announcement! Coming Fall: a new way to earn Rewards based on your Achievements,” a tweet from Xbox LIVE Rewards states.
No other information beyond the expected arrival period was provided.
The twitter account currently offers free MS Points for participating in other various ways on Xbox Live outside of earning achievements, so that's definitely one possibility for what rewards are being referred to.