Well that certainly was a...thorough, ending. Eren going for the Code Geass resolution. Followed by one grim fucking credit sequence.
And they used the solution I'd been suggesting since they introduced the euthanasia plan. "Just cure all the titans". No idea why that required killing 80% of the world first.
I'm glad they thought up something interesting for the final battle. Past Shifter Titans was interesting and still plausible.
Well they laid the groundwork for that. The kid walking into Eren's grave tree mirrors the tree Ymir became a Titan in.
So if they want to start the cycle all over again with kid with dog as Ymir and Eren as the space worm, they've set that up.
It's basically just there so Mikasa can resolve things. Seeing Mikasa kill Eren despite still loving him allows Ymir to kill Fritz in her mind.
I'm not sure whether it was that, or killing the worm, that cured everyone's titanitis.