Let's come up with scenarios that will make Armin look smart and give him the chance to save everyone.
I think there's a chance that when everyone meets in Shiganshina, Armin will remember when Eren transformed and protected them from a Canon ball. Then he will grab Mikasa, nuke-transform and eats all shifters present there (which will be all of them) Eren will be unconscious because he was near the nuke when Armin transformed. Then Armin eats him too, finally getting all Titan powers to himself.
Mikasa falls in love with Armin because he saved her (and because of her slave genes).
They unite the kingdoms of Paradis and the Asian Nation under his benevolent power. Mikasa becomes Queen.
Armin goes meet diplomats of the whole world and with his superior intellect convinces them that eldians are alright.
Armin also finds out how to lift the course of Ymr.
Last page is Armin holding his son, which he named Eren.