Jaime Lannister is a polarizing character: He pushed little Bran Stark out of a window, but he also leaped in front of a bear to save Brienne of Tarth. His waning moral compass aside, the GoT powers-that-be have long foreshadowed Jaime's ultimate choice of good over evil.
Think back all the way to season four, episode one, when Jaime and Ser Meryn go over the royal protection plan with King Jerk Joffrey. (You can watch the scene here.) Ever the little weasel, Joffrey sees this as the perfect opportunity to throw shade at Jaime and scolds him for getting captured and losing a hand. (Kids have ZERO appreciation these days.) Joffrey then pulls out the Book of Brothers (the record of all knights' accomplishments) and snidely remarks to Jaime, “Someone forgot to write down your great deeds.” The Kingslayer confidently responds, “There’s still time.” Being the brat he is, Joffrey zings back, “Is there? For a 40-year-old knight with one hand? How can you protect me with that?” Rude.
With that unpleasant exchange in mind, let's look back at the season seven finale. Jaime plans House Lannister's northern defense strategy, and Cersei waltzes in to inform him she plans to leave Jon Snow and Daenerys Targaryen in the cold (literally) instead of coming to their aid, per the truce agreement. Oh, and she wants to conquer their kingdoms while they fight the Army of the Dead, too. Jamie can't help but be more concerned about the fate of humanity and the imminent White Walkers attack than Cersei and her petty, delusional plans. He tries to reason with her but eventually storms off and leaves King's Landing on horseback. Chances are—with Cersei's increased mental instability and Jaime's newfound conviction to do the right thing—Jaime's off to join his brother Tyrion and his dragon-wielding friends in the north.
There's no guarantee that Jaime rode off to Winterfell to join Team Dany and Jon, but his “There’s still time” quip and the tiny snowflake drop on his gloved golden hand at the end of the season seven finale lead us to believe he will disobey Cersei for the greater good of the Seven Kingdoms.
We just don't want to be there when she finds out...yikes.