Submitting Steel/Grass
Weaknesses: Fighting, Fire
Resistances: Dragon, Electric, Fairy, Grass, Normal, Psychic, Rock, Steel, Water
Immunities: Poison
Hits SE: Ground, Rock(both STABs), Water, Ice, Fairy
Resisted by: Flying, Poison, Bug, Steel(both STABs) Fire (both STABS), Grass, Dragon, Water, Electric

Pros:
No weakness to VoltTurn, especially with resisting volt switch
Poison immunity
Only 2 weaknesses makes it extremely easy to come into attacks
Resists all 4 tapus, allowing you to trap them quite easily
Typing is a natural counter to Pelliper/kingdra, allowing you to have a natural rain counter that's somewhat easy to build around
Resists Electric and is neutral to ice, allowing you to have a good match up vs BoltBeam dependent pokes
Resists Rock and is neutral to ground, allowing you to have a good match up vs EdgeQuake dependent pokes
Best Tomo bait ever?
Cons:

Bad match up vs common defensive fire and fighting types, such as Mollux, Arghonaut and Pyroak
Can't touch steel type walls such as Skarmory without coverage
4x weakness to fire is a detriment to pokes who run BoltBeam+fire coverage such as Zapdos and Mega Manectric

Steel/Grass is an amazing defensive typing, shown by Ferrothorn in OU. A typing that's weak to some of the more common defensive pokes in the tier is somewhat unfortunate, but it does allow you to bait in Tomohawk extremely effectively, a Pokemon that we should be trying to trap. The only major pitfalls to this typing is its 4x weakness to fire, but that's one that can be remedied by teammates, and it makes the common BoltBeam less common in the future to Pokemon that need to pick their coverage, such as Tapu Koko, who can only pick HP Fire or Ice. If this typing were to become reality, I could see it easily paired with Zygarde, as they have amazing neutral coverage together, and their resists work incredibly well together as well.