Not having seen any spoilers for the finale airing tomorrow, I've no clue what's going to happen. But I have a few ideas...especially since it's been promised as an emotional roller coaster.
Rhys is gonna get killed. If Spike wants to hurt everyone near Jack, he'd have to be included...and Rhys certainly wouldn't hesitate to do what he could to protect Gwen.
To that end, someone in Torchwood will get offed. My money is on Ianto or Owen. Ianto is Jack's new boy, so Spike would be especially jealous of him. And Owen is on borrowed time as it is; whether or not he gets permanently killed by Spike, or simply runs out of whatever energy is keeping him reanimated, it seems likely he'll be taken down. And hey, he can't heal.
Grey, I have no idea on. It seems possible Spike would turn him on Jack somehow, and force Jack to kill his own brother...or possibly have Grey be the instrument of his revenge. I can't even speculate on that one with any certainty.
Not so much a prediction, but it would be great to see Bilis again. Just sayin.
Now ask me predictions for Doctor Who, and I have no clue. I demand more Cybermen though!