We've datamined the Mage Tier set bonuses for Patch 9.2 Eternity's End! This includes the 2 and 4 piece bonuses for Arcane, Fire and Frost.
Arcane Mage Tier Set Bonuses in Patch 9.2 NEW (2-Set) Increases your Arcane damage dealt to enemies affected by Touch of the Magi by 18%. NEW (4-Set) After consuming 8 Clearcasting your next Touch of the Magi is instant, incurs no cooldown and lasts for 6 sec. Read more on wowhead.com