At some point I'd meant to rec 100demons' futurefic "impossible blossom, to sweet impossible blossom"
(7,589 words; Teen and up; canon-typical violence). Jean was the one who had to deliver the terrible news about Marco to the Bodt family. Seven years later, he's a corporal with a drinking problem, Armin is a lieutenant commander…. and Kurt Bodt, Marco's little brother who resembles Marco quite a bit, wants to join the Survey Corps and avenge his family, which was destroyed by his brother's death.
Jean, who can't abide the thought of Kurt dying the same way Marco did, hits upon the solution of being as much of an asshole to him as possible, first to discourage him from joining up, then — after Armin deliberately assigns Kurt to Jean — to protect himself emotionally. Needless to say, things don't go the way Jean plans, but I'm not going to spoil anything for you.
I will add, however, that Armin is as well written as Jean here — more mature, more self-confident, proud of his work, and unwilling to take any shit off Jean whatsoever. Their working relationship, antagonistic on the surface but with great mutual respect at the core, is one of the delights of this story. Also, while 100demons did model Kurt's personality on Eren's, any similarities between future!Jean and canon!Levi are totally unintentional. (Oh, and yeah, there's Sasha, too, and she hasn't changed too much.)( There's more! But it might be a bit spoilery for the above-described story. Just go read that fic, then come back and click on this cut. You'll be glad you did. )