So, I've never really taken the time to write out a good answer to this question, so I hope you'll excuse the very long post. There's a lot to cover since all five of these stories have a lot going on in comparison to the original.>Mastermind in the Franxx
MitF is considered by some of us to be the new canon. It's that good. The first few chapters are rough, since olcon (the author) was out of practice. But now that he's got his flow going, he's pretty good... he still probably needs a keenly eyed editor, but it's a passion project, so some things like his reliance on adverbs in places can be excused. Basically, he isn't like a "George Orwell," but he's hardly a "Stephanie Meyer" in terms of skill.
Some people have complained of characterization, but, everyone's allowed to take characters in a different way. Hiro is by far the most changed, because hey, the boy's already a bit of crazy person and instead of just living the parasite life, he's spent the past 6-8 years ruminating on what happened and being tutored by Dr. Franxx to be a warrior-monk. The boy is gonna be a bit more hardcore than he was in the original.
There's also the matter of Zero Two, Mitsuru, and Ichigo. Zero Two basically jumps into her bubbly attitude from the get go, because well, why wouldn't she? Darling is with her. But, she's still an ice queen when he's not around and if she gets stressed out the "femme fatale" comes out to protect herself. Mitsuru meanwhile is just as edgy as he was, but he's privy to Hiro's secret and was trained by Dr. Franxx to be basically the sex ed/biology "teacher" for the squad. This of course leads to problems because Mitsuru still has some sort of issue with Hiro and is still his all-too-considerate self. Ichigo meanwhile basically gets removed from the love triangle immediately because Hiro doesn't want her fucking things up. How she gets removed drastically alters her behavior towards Zero Two and Hiro early on.