>Isayama stated that he wants the ending of Attack on Titan to be like that of Guardians of the Galaxy Volume 1
Lets break this down into some possibilities.
First, ignoring the dance off shit, the ending of GOTG 1 is basically this
>powerful guy acquires one of the strongest powers in the universe
>bunch of characters that dont really like each other that much have to team up to take him down (alternatively, more than one power has to come together to stop an even bigger power)
>they succeed and peace is restored temporarily
>(sooner or later, an even more powerful force, in this case thanos, will appear and make things even worse than they ever were)
Now lets apply this to SnK.
The powerful antagonistic force is Eren (since he has the founder, warhammer and attack titan) and ultimately, the eldians and marleyans (or probably the entire world) has to bury their conflicts and come together to stop Eren from destroying the world. They succeed, Mikasa finally lets go of Eren, Armin survives long enough so that he can fulfill his role as narrator of the story and Eldians make peace with the world. Though since there is memory timetravel stuff, the ending ultimately might not even mean anything, since it can be "reset", so we leave on a bittersweat note.
Instead of Marley and Eldia coming together, its the titan shifters that come together to fend off the entire world, or Eren just eats them all and becomes the ultimate being to safe his people.
Maybe in eating all 9 titans he gets access to the devil and makes a deal to get rid of all the titan powers altogether, so eldians would never be a threat anymore and peace might be restored. But even then, the world probably won't forgive their sins even if they are regular humans now, so 100% peace will never be achieved.
Maybe it even is a mix of both. I definetely can see Isayama getting rid of the curse at least and letting Armin survive long past his 13 year time limit.