So here's my theory:
Sakurai knew Byleth would make people mad. After all, he was well aware of the backlash Corrin got, so he planned to make Byleth have a varied move set to help with some of the rage. Keep Corrin in mind for a second.
Sakurai also realized that in order to get Byleth's presentation translated and edited in time, he would have two options: record before the holidays, or record right after everyone came back from the holidays. If he did the latter, Byleth wouldn't release until most likely late February, and the longer the wait in between characters the more disappointment people would have.
However, Sakurai realized he could kill two birds with one stone here. If he does the presentation in November, not only could he have time to prepare for Byleth and polish some things, but he could also have time to make a new trailer for the next available Nintendo Direct. So he gets in contact with Nintendo and plans for Byleth's reveal to be situated nearby the first Nintendo Direct of 2020, all so he can quell the anger of irate fans.
And the best part is: He's already done this before with Corrin. He knew as soon as Nintendo told him "Please add Corrin for Smash Bros DLC" that people would not be elated, so he planned to reveal him at the same time as he revealed Bayonetta. I think the same applies with this situation. Nintendo asked for him to include Byleth, so he knew he would have to time it properly again so people weren't as angry with his inclusion. He knew he couldn't include him in slots 1, 2, or 3 because Three Houses hadn't been released yet, and if he released him in slot 4, very few people would be excited for fighter 5 since it would be Terry Bogard instead. The releases planned themselves.
TL;DR, Sakurai knew Byleth would make people angry, so he's planned Byleth's release around the first Nintendo Direct of 2020. Fighter 6 will be revealed there, it will be a fan favorite inclusion, and it will all be to restore faith for fighter pass 2. However, I don't think fighter 6 will release until later, much akin to Joker.