I had horrible thoughts while I was gone...
I was thinking on the idea of a 'new' Master. OC was what first popped into my mind, but then I remembered: good old Sacchin. Yep, use Satsuki from Tsukihime. Since she
did get vamped and yet didn't actually get dealt with in the Arc/Ciel arcs, she's available to drift on over to Fuyuki in time for Fate. And she'd probably be the only character in existence other than Sakura with enough raw fricken magical power to fuel a Nanoha character, and since she doesn't really know how to
use it, she can be kept from being an overpowered sorceress. And she actually has a perfect Assassin type lined up
Of course, Satsuki summoning Shiki present a problem - too
much ownage. I mean, Shiki versus Saber = Excalibur chopped in half and some serious problems down the line as far as plot's concerned...
So, for the moment sacrificing the humour of that idea, I go farther afield, to Shiki's
father. (Of course, Satsuki wouldn't have a clue of that, since as far as she knows, Shiki's father is Makihisa Tohno, not Kiri Nanaya) He'd be much better for keeping things balanced, because while he's probably more
skilled than Shiki is, he also doesn't have the Mystic Eyes of Kill Anything You Can Poke.
That would end up with a strange, strange team, where the close-combat fighter stays out of direct combat (lacking both staying power and the hitting power to do much without surprise), the Master is the meat shield (vampire regen, gotta love it), and the Caster tends towards melee smackfests.
You'd have to twist how the Grail workd in summoning heroic spirits for it to summon any Nanoha girl, since they are essentially fctional beings, unless you use the excuse of how Kojiro was made - and that can have adverse consequences.
Well, this one's assuming that they're actually fictional, and not just living in another world. Crossovers don't
normally assume that one is real and the other is still anime, right?
Although, actually summoning her would be a wee bit more difficult.
Mad coincidence is the ultimate plot device.
Still, I find Nanoha's magic to be completely !?!? in terms of trying to mix it in with the Nasuverse. Granted, it probably wouldn't be impossible, but considering that most of Nanoha's magic involves lots of HUGE freaking explosions, it makes me wonder... In terms of how much mana that must be consumed to actually wield that much power, Fate (or whatever other character you might use) would have to either be a true magician, or have such levels of deep mana reserves it wouldn't even be funny.
Yeah. They
do have insanely overblown magical reserves even for Nanoha canon. And that could be a very effective plot point - she requires huge fsking amounts of mana to actually perform at that 'utter pwnage' level, so...