Theory of beast titan; Was once a human. Obviously.

Non-manga reader theory

It's starting to look like any and all titans were once humans, some can just actually transform back and control the "urge" to eat humans (erin being a prime example). If we look at the notebook OVA, it's quite obvious titans have this HUGE sense of urge to eat people, despite trying not to eat the girl the titan couldn't resis it.

Also the titan says "ymir's people" which didn't become relevant until now, this leads a lot of question to ymir now. Does ymir's people mean all of humanity? If so why specifically ymir, is she some immortal that was here when the titan first appeared? possible. Whatever it was meant, it's more than likely we are going to get some deep backstory with her in this season.

It also looks like christa family is part of the church? or connected in some way.

The one thing i keep trying to figure out is the ED, specifically on the 3 little girls eating flesh, and what looks to be in a church as well. I wonder if how you become a titan is actually just eating human flesh or maybe a specific type of human? Which could explain the titans urge to keep eating humans.

Also i believe the big ape titan was maybe the first of all titans? He is the only one with a bunch of hair, he is absolutely huge as well. Also in the ED we see him, a dinosaur, some mythical creatures roaming around tramping over everything. But titans only appeared 107 years ago. So unless titans existed for longer, he was possible the only titan in existence for an extremely long time until 1 day they started mysteriously popping up. Also to back my theory up, this titan that seems to be intelligent and can speak, had no idea what weapons the people were using, which is very odd. Because if he was a person living inside the wall he would obviously know the weapons, this confirms he has never been close to the wall until now.