Crossovers that shoould be: beginings of Sub Genius

[Necratoid]

Yes. The main character's little brother, in fact.
Also, yes, yes it would.

Speaking as someone avoiding the canon for now... I think your off on the casting.  Nanoha as the player character sure... but I've never seen a version of Hyatt that considers the wheel chair and soul eating thing to be anything, but a cost of doing business and something that makes her pack happy.  I see her more as the girl running a store and going on her own missions for supplies.  A random Knight left tending the store.  I can't see her as the psycho one, it'd be her pack behind her back trying to keep the world around.  Though with all the crazy NPCs and monster around... I can see them filling the book quietly in the back ground.
The crazy one trying to keep the world around at most costs is Fate.  The one were her sister is up and vital and Queen Mommy of the Law Guard fails to operate under the child raising theory of, 'The beatings will continue until moral improves!", at best.
I'd also suggest that  Nanoha's rich friends end up as part of her guild or in the rival with sanity role.  I can se at least one of them as being a beast tamer ignoring the species restrictions.  Their archer bunny girl misnion ever confused if she counts as a minion, pet, or romantic intrest with that one.  Not something that effects their working relationship... but confusing on the personal level.  The other off worlder in the clan is forcefully ignorant about what is going on with that.
I also wouldn't make Nanoha's clan the TSAB one... they'd be baffled in orbit about how the protocol handled this kind of thing.  The jewel seeds, two major, reality altering books and the general multi layered reality of that world with random massive seals breaking at screwing up any hope of orbital scans doing anything productive.  They'd need a ground clan to get anything done.  I can't imagine the paperwork this would entail.
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If you wanted to go for the trifecta here... have Fate (or alternately Nanoha's two normal friend) end up bringing in a certain mysterious play into this mess.  That would screw things up nicely.
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Either way you do realise that you have doomed an entire city once a Marlboro snags a jewel seed and turns into its Final Fantasy 7 incarnation.  Sneak Attack>Bad Breath>TPK.