The last time Tyrion and Sansa are together, they're unconsummated man and wife living in the Red Keep. Since then, they've each gone on the run and endured a lot — particularly Sansa — but each has come into a certain degree of influence as advisers to their own respective rulers. Now that Jon has bent the knee to Daenerys and the entire Targaryen camp is headed to Winterfell to prepare for the Great War, Sansa and Tyrion are bound to reunite.
Based on how Tyrion speaks to Jon about Sansa at Dragonstone in season seven, it seems that there is no ill-will between the former spouses; perhaps they will serve as trusted advisers together, bringing a sense of level-headedness and much-needed diplomacy into Daenerys and Jon's throne room.
