What if Falco ate Reiner?

The events of the story would remain the same up to Marley’s counterattack in Shiganshina.  When Marley invades Paradis to retake the Founding Titan, along with rescuing Warrior candidates Gabi Braun and Falco Grice.  In the main story, Falco is turned into a Pure Titan by Zeke, and almost eats Reiner, until Porco gains his attention, causing Falco to eat and inherit the Jaw Titan instead.  Falco's flying Titan would go on to play a pivotal role in preventing Eren from destroying the world.  However, in this timeline, Falco ignores Porco and eats Reiner, gaining the Armored Titan.   

There would be surprise and confusion all around.  Much of the battle would play out the same, but with one major change: Gabi dies as Eren's Founding Titan crushes her since Reiner wasn't alive to carry her off.  Instead, Porco grabs Falco and hides inside the city, like Reiner and Gabi did.  Annie would still be freed from her encasement, but wouldn't meet up with the others since Connie never took Falco.

Hange and Levi would still meet up with Pieck and Magath, and would carry out the plan of capturing Jean, Onyankopon, and Yelena.  Being in Shinganshina, a healed Porco and Falco would notice the Cart Titan, and follow them, meeting up with the Alliance.  Similar events would follow, with the Alliance understanding one another, and fighting Floch and the Yeagarists at the docks (Porco replaces Reiner in this battle).  Falco would transform into the Armored Titan, which still takes on bird-like characteristics since he was injected with Zeke's spinal fluid.  Since he's the Armored Titan, Porco uses his Jaws to free Falco.

During the final battle, events continue the same.  Even alone on the ship, Falco realizes that he can fly since he ingested Zeke's spinal fluid, and flies towards the others with his bird Armored Titan.  Events of the final battle would roughly be the same, with a few minor changes.  Armin is freed from the Pig Titan by Porco, who is able to catch up to it thanks to the Jaw's speed.  Porco and Pieck would hold the centipede creature back, trying to stop it from reaching Eren.  From then on, the ending would be the same, but Porco would be part of the ambassadors who are trying to make peace with the world, though he has a more pessimistic tone to the idea.

Meanwhile, Annie would be heartbroken, believing that her father is dead, and would give herself up to Paradis for her crimes.