I'm not completely convinced Supergirl leaving for the future is the best end to the series. With her leaving Catco and in light of her argument with Alex, I'm wondering if her perfect ending is going public with her identity. It's not as dramatic as her leaving or losing her powers or even dying, but it would close a chapter on her life and it would emphasize how important it is for aliens on Earth to be comfortable in who they are.
As for Kara and Lena, I think the point of their story wasn't to build up romantic tension but to make us question if a Super and a Luthor could learn to trust and rely on each other. The biggest turning point was Lena finding out Supergirl's identity and then the fallout of that. Them remaining friends and Lena proving herself to be on the side of the good guys was a compelling enough story without it ending with them being in a relationship.
Part of me wants Lex's threat to be over for good, the other part wants him to show up on Superman and Lois.
I definitely liked William more once he stopped being a potential love interest. He was never my favorite character, but I liked him enough to think he didn't deserve to go out the way he did. It was a sad moment.
Her secret identity and whatever career they could have given her suffers tremendously when practically all the supporting characters are superheroes who are aware of her dual identity.
Cat Grant was one of the best parts of the entire series.
And did they get rid of some actors because of political differences? That feels unprofessional.