If I had to guess:
1. Bonus miles (already in place)
2. Free checked bags
3. Early boarding
4. Priority Security
5. Free same-day flight changes
6. Ability to bring a checked bag on Blue Basic.
That's most of the benefits of Mosaic; my guess is that B6 will save EMS seats at the airport and free in-flight drinks for its own elites.
Doubt B6 will agree to most of those on Blue Basic fares; B6's regular Blue fares are much higher than AA these days. For example, AA is charging $49 for Main whilst B6 is charging $114 for Blue - MIA BOS.