There was another big ending theory I read a little while back suggesting that Eren was somehow a reincarnation of Ymir. I've been re-reading chapters with that theory in mind, and found a bit that fits nicely.

When Eren is talking to the alliance, he says "In order to gain my own freedom... ...I will take freedom away from the world." That sounds a lot like what Eren told Ymir to do, and we don't know if it's really Eren that's saying that. This is immediately before we see child Eren and Ymir standing right next to each other with the same expression.

While I like the Eren = Ymir theory, I'm not sure if I totally buy it; just because there may not be a lot of room to explain it all (unless we get a major info-dump chapter). I'm sort of looking at this with my GoT PTSD, when Alt Shift X kept going on and on about a prophesy that sounded cool but the show never referenced in any way. He kept bringing it up right up to the only episode it could be relevant, and there was no way it was going to happen because there wasn't enough time to make it make sense (also D&D needed to ruin it in other dumb ways). But Isayama is a much better storyteller (than D&D), so he could possibly pull this off.