I feel Shauna will get the Kalos starter weak to yours, the other MC character will get the Kalos starter super-effective against yours, Tierno will get the Kanto starter weak to yours, and Trevor will get the Knato starters super-effective against yours.
My reasoning is Tierno and Trevor already have Pokemon before the other 3 (including you) get yours, so maybe Sycamore already gave them the Kanto starters before. The MC character seems like the more serious rival who will try to be better than you, while Shauna seems more like a happy-go-lucky trainer. Trevor seems like Cheren, while Tierno seems like a male Bianca --- I sleep... at NIGHT. 3DS FC: 1864-8493-9429