That's unfortunate I liked this show. They should replace it with one about Powergirl and reuse some of this cast. Let the hunt for a young Pamela Anderson lookalike begin!
So, how would you end it?
1. Kara is exposed to aquamarine Kryptonite invented by Lex Luthor and splits into 2 different entities, one human, the other the ultimate super powerful Kryptonian. Her Kryptonian side goes insane with power and her human half eventually has to kill her using regular Kryptonite, fully expecting to die as a consequence. Instead Lena is able to save her and Kara now gets to live her life as an ordinary human without the burden of being Supergirl. In the final scene Kara goes to her desk at Catco and reports on a new crisis, realising that even without her powers she is still a hero.
2. Kara receives strange dreams of a lost Kryptonian civilization in deep space. She decides to journey into the cosmos to rescue it, leaving her friends and family in the care of Powergirl who has arrived on the scene.
3. Kara fights an almighty foe and eventually triumphs but at the cost of Martian Manhunter's life. In the final scene Brainy confronts her and asks why MM is impersonating Supergirl? MM explains that the real Supergirl is dead but he can take on her role and avoid her family from the grief of her loss, the legend more important than the woman herself.
4. Lex Luthor artificially impregnates Supergirl in an attempt to create human/Krypton hybrids. The gang defeat him but Kara still gives birth to triplet daughters and retires from the superheroine game to move in with her mum and raise her kids. In the final scene her mother draws her attention to her babies who are now demonstrating the ability to hover under their own power.
5. Kara is finally defeated by a foe and dies. She wakes up in a mental asylum, tells her doctor and nurse (Jon Cryer and Katie McGrath) that she is ready to go home now and returns to her family (Dean Cain, Lynda Carter and Chyler Leigh). In the final scene she sits down and begins to write about her delusion, turning it into a comic book.