No.6, Owari no Seraph Recommendations
Both animes are about persons fighting a system they're captured in. Both systems are inhumane and cruel. Also the males have a deep friendship. No. 6 is not shounen ai romance but still has a BL subplot. Owari doesn't have such so far and I guess it won't since it's shounen. Owari is of very high quality (animation, music, atmosphere, story) and can bring your tears out. No. 6 has a very good manga/novel but the anime (is a rather stumpy) can't compete with the manga/novel in the slightest. Although Owari isn't shoujo it can be watched even if you prefer shoujo over shounen.
Both take place in post-apocalyptic worlds. Foreign species threaten all of humanity.
Both series have definite BL elements as well, but No.6 is a lot more explicit with their relationships. It's fluff in a dangerous world all over ^^
Both talk about a type of virus affecting a nation. Both have a sort of "war" between two different populations. Also has a protagonist that faces internal conflict because they have involvement with "the enemy population".
(Also has hints of shounen-ai)
These are two of my most favorite animes that I am most passionate about. I watched them within two years of each other and although the overall plot is different, the characters and situations are not.
Like Nezumi and Shion, Mikaela and Yuuichiro were separated at the age of 12. Mikaela is much like Nezumi in a way and Shion much like Yuuichiro, Mr. There's Always A Third Option.
In Owari No Seraph, fans find themselves either shipping Mikayuu or Yuunoah, like in No.6 how fans find themselves shipping either Nezushi or Safu x Shion.
In both series, one of the two characters has a goal