Maybe I'm a brainlet for just now making this connection but
>months ago Isayama showed us the final panel, seemingly confident we wouldn't understand it fully despite it seeming straight forward
>someone who looks like Eren clutches a baby and tells it "You are free"
>Eren meets Ymir this chapter, and is compassionate towards her, even looking a little sad
>we get the plot twist that Ymir is actually nothing but a slave without her own will
Is this set up for the reincarnation theory? Ymir, the origin of royal blood freed by Eren, and reborn from her own descendant into a new world. A chance to live a real life of her own after Eren has fundamentally changed it for the better.