Alright Comartemis, ignore ragnarok for a moment.
Think "How can I develop the Fate/Nanoha relationship in such a way to be three-dimensional?"
Sailor Moon was very explicit about Usagi's relationship with Mamoru, and the trials they went through. It developed both characters nicely.
Nanoha, by comparison, showed us why Fate and Nanoha ended up friends. . . but refused to go any further. Oh, sure we can reasonably assume at this point that they're a couple, but the Nanoha franchise failed to give their relationship any sort of depth. All dialogues, verbal or otherwise, about all the important things in a relationship: sex, love, support, commitment, issues with finance, homosexuality, personality: apparently it all happened off-screen off-screen as far as the Nanoha franchise was concerned. We're just supposed to accept that all is well, and oh how they look cute together. They might as well be both asexual people who are comfortable enough with each other to live together.
And it's not just Nanoha/Fate; ALL relationships in the Nanoha franchise are like that. We have little explicit material of the issues Fate confronted about adjusting to her new family, or Chrono and Amy's marriage, or why Yuuno just dropped off the radar, or how well Hayate adjusted to her new family. The Sound Stages helped a bit, but not enough, which means that relationships in the Nanoha-verse were kinda flat.
Frankly, it was unsatisfying. Most of the details about the Nanohaverse's relationships are fanon, because canon offered so little.
But now, you're taking the Nanoha characters into a shoujo series. And shoujo is nothing, nothing I say, without explicit dialogue of emotions, feelings and personality conflicts, hostile and otherwise, and the drama involved. So, you have to do better. And the worst thing you can do is focus on the Nanoha/Fate romance to the exclusion of their relationship development with other people. The second worst thing you can do is to ignore romance almost entirely as canon Nanoha did and just go "Oh they ended up together; here's some moe images to make up for lack of actual development".