Darling in the Franxx is wonderful, the art is amazing, the story if not original is pretty good the characters are varied... at least up until it goes to pot. As enough reviews already exist for this criticizing the ending, I want to suggest an alternative, which I consider, at least in my mind, to be canon simply because I said so. There will be spoilers. We begin just before the battle at the Grand Crevasse.
In this world, people are no longer born, rather they are grown under APE's supervision. APE's goal is to create "perfect humans," with absolute obedience, incredible war potential and no emotion or gender. They accomplish this by modifying humans, infusing them with Klaxosaur blood in post conception development. Zero Two is a result of these experiments, she is a powerful weapon but retained all of her "undesirable" human traits. She was originally thought worth keeping only as a curiosity. The Nines are later experiments considered nearly "perfect humans." They have strong war potential, perfect obedience, no gender and are nearly emotionless save for blood lust.
Under the guise of trying to wipe out the Klaxosaur, Squad 13 is sent to the Grand Crevasse, a Klaxosaur nest. Squad 13 is a threat to the project of creating "perfect" humans and the real goal is to have them die in an "accident." However, the battle goes too well. Thanks to Squad 13's "undesirable" human traits, they have become so skilled that though they are placed on a front line, they are not overwhelmed by the Klaxosaurs. APE abandons subtlety.
The battle continues. The Nines, using skilled combat, direct a large number of Klaxosaurs towards Kokoro and Mitsuru who struggle to pilot due to Kokoro's pregnancy. In the stress, they regain their memories and replace their rings on their fingers and continue with renewed strength. The Nines worry that Kokoro and Mitsuru will survive. One of them takes initiative and attacks them leaving Mitsuru dead and Kokoro stranded in the damaged Franxx and believed dead. The day ends and both Klaxosaurs and Franxx retreating before the night.
That night, Squad 13 is lied to, they are told that Mitsuru was killed by their negligence and they are blamed for not crushing the Klaxosaurs the day before, they are emotionally and physically drained. Later that night, they are contacted by Kokoro in her damaged Franxx. Through tears, she relays a message of what really occurred. Disgusted with APE, Squad 13 plans to fight against the Nines free themselves.
The next day, Squad 13 feigns to barely hold their line against the Klaxosaurs, citing their missing member, and requests the assistance of the Nines. Two Nines are sent to support them. When the Nines engage in combat with the Klaxosaurs, Squad 13 attacks them from behind. A fierce fight ensues. The two Nines fall in heaps of torn metal. Squad 13 prevails because of its humanity. Seeing the destruction of their comrades and being outnumbered, the other Nines flee.
The children's caretaker Nana snaps under the strain. She convinces her partner to join the children in their fight for humanity and take control of the plantation for their sakes. The Nines and Squad 13 each call on the remaining squads and plantations to rally for what they each deem humanity. Some join squad 13 and others join the Nines out of blind loyalty. A battle ensues between these two factions resulting in the solidifying of a group of rogue plantations and squads that fight against APE and the Nines who take their followers and flee to rally all of APE against Squad 13.
APE continues to build more "perfect" humans while Squad 13 tries to develop a human society. The next season focuses on what an ideal humanity is, a homogenous, changeless, emotionless, obedient, waring, genderless weapon, or a diverse, changing, driven, thinking people.
I think that this ending would have done it for Darling in the Franxx. You tell me if you agree.