Satomine Night
The Power of Z!
Given Lusamine's obsession with them I'd say they will make an appearance in the main storyline
I agree. Thanks to Lusamine's obsession with them, the Ultra Beasts are a major part of the storyline, and unlike with the XY legendaries (Xerneas/Yveltal being replaced with Zygarde), they can't be as easily replaced. So, I expect the Ultra Beasts to be in the main series, and not just relegated to a movie or two, and I expect Ash to join in the battle against them.