Koutetsujou no Kabaneri, Shingeki no Kyojin Season 3 Part 2 Recommendations
Both have similar realisator, both show humanity struggling to survive against a very strong ennemy. And both have mysteries about the world itself.
If you're looking for an anime to watch waiting for attack on titan's final season, Koutetsujou no Kabaneri is a great choice. The two anime are really similar, both worlds are in a world where humans are on the brink of extinction.