See that face? Tyrion is up to something.
All throughout the series, both in the show and books, Tyrion’s greatest struggle is wanting his family to accept him. All of his major life events revolve around this fact. He nearly had a wife, but his father disapproved and then fabricated a reason to have his wife gang raped then sent away. His nephew died before his eyes, and his sister blamed him for it for virtually no reason, and then his father sentenced him to death despite knowing Tyrion was innocent. Tyrion never knew his mother because she died giving birth to him, and of course his family blamed him for that. Shae, his lover and only real chance at feeling accepted, betrayed him. Oh, and Tyrion killed his father, and all his remaining family hate him for it.
Finally, at the end of Season 7, his sister says she still blames him for ALL her kids’ deaths. She has a chance to kill him like she’s always wanted, but she refrains for some reason. Then (pay attention), Tyrion figures out that she’s pregnant, and then the scene ends. Yep. It just ends. Cersei allows her enemies to leave King’s Landing no fuss, and allows Tyrion to leave with them. Kind of strange, don’t you think?
With his sister pregnant, there’s just no chance that Tyrion isn’t going to take that seriously. A chance for the entire family to survive into future generations, a chance for Tyrion to prove his worth to his queen sister, a chance for Tyrion to serve his family in its most dire time ever.
I think Tyrion is tasked with killing both Dany and Jon (whether by leading them into a trap, poisoning them, or slitting their throats in their sleep), and Cersei believes all their forces will unite behind her against the White Walkers (with the Golden Company, apparently). After all, Tyrion is very close to Dany and Jon, and they are Cersei’s greatest and only threats besides the Walkers.
It only makes sense.
I’m not 100% certain Tyrion will try this, I just find it highly unlikely that he won’t.