Perhaps this is what Isayama had in mind for that video:That's most likely what was going to happen originally, Linked Horizon would get insider info from Isayama and put bits of foreshadowing into his cover art and videos. The most explicit being this:
Though the difference being this Eren has a kid and the kid keeps him from killing himself out of guilt. Yes he's a fucking bird here too.
After the Rumbling, Eren returns to Mikasa as a bird. They finally get together and a year later Mikasa lays an egg with Eren's child inside it.
Meanwhile, an upset world begins plotting to destroy Paradis. They gather their remaining forces and carpet bomb the island leaving many casualties. Bird Eren and Mikasa avoid this because they happen to be on their honeymoon in the country Mikasa's ancestor came from. All is not lost however, since all the casualties come back as birds.
The birds are angry and wage a furious bird war on the remnants of civilization. Flocks of birds swarm and caw at their enemies and eat what crops they can grow. The people fight back, but the birds' initial strike on their crops wounded them considerably.
Bird Eren and Mikasa's egg hatches. It's a little bird person like you see in the video. As the years pass tensions continue to rise, more bird people are hatched as the reincarnated Bird Eldians marry and have children with humans. Ymir the Founder sees this and tries to give the rest of humanity a way to fight back by turning them into snake people like you see in the video.
As a decade or two passes the fighting continues. Bird Eren comes to give a speech to the bird army defending the island. "If you we win we live, if we lose we die," he squawks, "We can't win if we don't fight." Both the birds and bird people understand him, since they speak bird.
The crowd erupts into a frenzy of excited squawking and cawing. There is a raucous applause as wings furiously beat the air, and feathers fly everywhere.
As generations pass, the snake people and bird people still fight with one another, but manage to create separate societies. The snakes live on the ground, but birds live in the air as birds should. The birds have cities suspended high above the ground where bird families live in giant birdhouses. Perhaps one day the snakes and birds can learn to live in peace.