We've datamined the Shaman Tier set bonuses for Patch 9.2 Eternity's End! This includes the 2 and 4 piece bonuses for Elemental, Enhancement and Restoration.
Elemental Shaman Tier Set Bonuses in Patch 9.2 NEW (2-Set) While your Storm Elemental or Fire Elemental is active, your Lava Burst deals 20% additional damage and you gain Lava Surge every 8 sec. NEW (4-Set) Casting Lava Burst extends the duration of your Storm Elemental and Fire Elemental by 1.5 sec. Read more on wowhead.com