I think Curtis might get immortality. Since that'd mean he could see ghosts and then see Nikki D: But I like Nathan with immortality. Maybe he'll get a new one.
Plus Alisha's power is gone as is Curtis's. So 2 new powers are bound to be coming, but then again Alisha didn't like having a power because she wanted to be a normal couple with Simon. So maybe she just keeps her share of the money and not get a power?
Plus I think the alternate time line might come back in the next series, maybe a future future simon comes back to explain to present Simon wtf happened (: