Crossovers that shoould be: beginings of Sub Genius

[Sirrocco]

I've got this idea going in my head, and I'm trying to work it around...

There are a lot of Familiar of Zero fics out there, based around the obvious crossover point.  Obvious crossover point is obvious.  I sort of want to turn it around, and pull Louise into some other universe through *it's* obvious crossover point, just to muck with things (particularly the shortly pre-familiar Louise, so that she can try summoning a familiar in this strange new world, and see what she gets).  The problem is, the only other obvious crossover point I can think of is the Buffy Halloween Fic thing.  The obvious person for the role would be Willow, and making the familiar ritual work would be relatively easy to handwave given her in-canon magical powers, but I can't see how it would result in a premise that was all that interesting - most of the characters you'd think to pick have all been powered up and shipped before.  Gunn, maybe?  Possibly a slayer, for having the familiar powerup do absolutely nothing.

So - can anyone think of a Louise/Familiar pairing off of a Halloween fic base that might retain some scrap of interesting and unexplored implications that make it worth more than a short one-shot?  Alternately, what other obvious crossover points are there in other universes that might result in something more interesting?

Okay, actually, a double-crossover where Willow halloweens as Louise and then does the summons and actually manages to get Louise could be pretty entertaining, if only for how mindbendy it would be for her.