Even though Brienne currently resides at Winterfell as bodyguard to Sansa Stark, we refuse to believe the series would leave her out of the final battle (or end with her pining over a man). Brienne is a warrior, so it only makes sense that she joins Daenerys in the fight for King's Landing, especially after Euron and Cersei succeed in killing a dragon, destroying a fleet, and beheading Missandei.
While it would be cruel of Daenerys to use Brienne as a weapon against the queen, Brienne certainly has a massive personal reason to storm into the Red Keep and cut down all those in her path — Jaime Lannister. In our minds, Brienne and Jaime's courtship ends so abruptly that there must be more to the story. We believe the pair will meet again on the battlefield in a scenario where Brienne fights the Mountain to kill Cersei or save Jaime. Such a battle would not only provide closure to Brienne's side of the relationship but also fulfil Maggy the Frog's prophecy, first revealed in season five, which states that someone "younger, more beautiful" will cast Cersei down and take all she holds dear.
Jaime and the Mountain are the only people of consequence left in Cersei's life (other than her unborn child), and Brienne the Beauty holds the upper hand in both the matters of love and warfare. Remember, Brienne is one of the few people to defeat Sandor "the Hound" Clegane in combat, which means she's a formidable opponent for the Mountain. She is also the more honourable of the two women when it comes to doing what's best for the people of Westeros and the people she loves.