I really have no idea where Decidueye will be this Generation.
He is pursuit weak, weak to knock off, defenses that are pretty good but doesn't compensate well for his low speed. OU threats like Bisharp, Weavile, Mega Pinsir, and Scizor are dangerous against him, which really sucks.
I can see him do well against pokemon like Breloom, Ferrothorn, Amoonguss, Rotom-w, Slowbro, and M-Loppuny though.
He seems to perform well theoretically in UU. Checks Celebi, Gardevoir, Vaporeon, Chesnaught, and M-Sceptile (somewhat)
A slow pivot that can trap foes and set up baton passes can potentially be really good next generation, who knows what's gonna be relevant then.