The rewards update was leaked earlier and now has been confirmed by Kabam.
6-Star Nexus Crystal and Rank 4 Material Added
This article earlier discussed the leaked image that showed rewards. However, Kabam has just posted the detail on the forum and that leaked image with reward was true. So now Master Alliances will get 6-Star Rank 4 Upgrade material and Platinum 1 and Master Alliances will also receive 6-star Nexus crystal. These are not the only addition, lower-tier alliances will also get better rewards. You can have a look at the image above that shows rewards for all-tier alliances.