Game Of Thrones Season 6 Finale: 12 Last Minute Rumours You Need To Know
3. Sansa 4 Littlefinger

One of the more popular theories for this season suggests that Sansa will be haunted by Ramsay Bolton thanks to his offspring sitting in her womb. Given his past, a Bolton child would be disastrous (perhaps later becoming a rallying point for Bolton loyalists) so Sansa needs to keep the secret to herself.
One rumours suggests she will do so by marrying Littlefinger, who we've already seen will come for his "reward" in the finale. He has further aspirations for power and becoming The King Of The North would suit his aims (since Jon isn't yet able).
Evidence?
The pregnancy thing is up for debate, but the teaser certainly suggests that Littlefinger already has a very specific plan for him and Sansa. There was something hauntingly lascivious about the way he approaches her, and there can only be union on his mind (as if it needed more emphasis they're by the tree that Sansa and Ramsay were married under).
We know Littlefinger wanted her mother, and we know he's not against faking personal relationships to gain more power, so this would absolutely fit.