NOOO. NOT our lovely Lena Luthor !!!

Haven't you considered an open ending with Kara (project supported by Brainiac) asking Lena to accompany her in the future, where Luthor's and cleverness could be used by the Legion of Heroes in their fights. Furthermore, Lena being a scientist with a great intellectual curiosity, this would be nirvana for her: working in middle of super heroes benefiting from powers and means going beyond her imagination and her personal experiences.
Several scenario are possible:
- the most popular: Kara/Supergirl decide for whatever reason to join the Legion of Heroes and fight in the future, leaving Alex, her foster Mum and friends behind. And Alex/Kelly getting married and adopting a child => I don't see Kara neglecting her role as aunt, who would spoil her niece or nephew!
Anyway, the writers will have to find something solid to justify the Kara leaving everything dearest to her heart behind her, if this is the chosen scenario.
- the less popular: Kara deciding to retire and giving up her powers to finally live like any human => I don't believe it too much.
Melissa Benoist promised a happy ending for her character so, let's just hope we won't be disappointed.