Kara and Mon-El deserve to be together.

Kara Danvers and Mon-El had one of Supergirl's most powerful connections. Thus, it made sense to form a bond, especially based on how they worked through their issues. For example, season 2's chemistry is very toxic and human. Thus, they were two flawed beings that had to learn how to learn from one another. For example, Mon-El started as a spoiled, immature, arrogant prince who believed he was entitled to his power and position. Thus, he did not see Kara's strength and values by the end of season 2 and became the man who both respected what she stood for as well as ultimately, as a person, sought to better himself. On the flip side, Kara learned how to see beyond the initial impression of him as a spoiled prince; she understood that he could be more. They challenged each other in the process, but by the end of season 2, they were each other's idea of a team. Thus, when Mon-El came back in season 3, his character growth suggested that he'd become a hero and leader that would be taking Kara's humanitarian compassion with him, in whatever time, whatever place. It didn't matter that Imra and he didn't come back to Earth in the same time. Mon-El's tie to Kara, his love for her and respect for her happiness was on the line—no question. Likewise, Kara had her life and her feelings for Mon-El in the future, always aware of the fact that their connection—unbound in that time—was legitimizing their bond. The scene in the DEO in the garden before the alien ship crash is testament to how they felt about each other; it was a scene where Kara—and Mon-El—literally had their reunion, and in a moment that should've almost bound them together again had there been no outside obligation, for the series to tease and take it away as something that could have been their rekindling was cruel. And because the writers couldn’t properly write or comprehend their relationship which only irked the audience more because everything was already established for an appropriate rekindling since they both worked on each other and bettered their lives. That’s why these characters who would have reunited should have had the chance, for their reunion was symbolic of relationship growth and reconciliation.