Hmmm, well, coming from someone who attempted to write a Power Rangers/Xena crossover fic once, I can't exactly say anything disparaging, now can I? (Hey, I thought it was a neat concept to steal the "Amazon High" premise and suck all the Pink Rangers back in time, dumping them into an Amazon tribe.) I had also, at one point, tried to do a Power Rangers/Sliders crossover. Didn't work out quite so well. (Hey! I was fourteen!)There's actually a rather interesting Buffy/Harry Potter crossover fic getting posted at the SlayerPups list lately.
Basically, what it comes down to for me is this: if you can come up with a solid, believable premise for how the two series' characters meet and interact, then it's fair game, really. Gratuitous crossovers just because someone happens to like both shows and shoves the two casts together for fun? Eh. That only works if you're deliberately playing it for laughs and amusement value.
But hey, this is Zahir we're talking about! I'm sure you'll do it right.
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"But if I weren't leaving you, I don't know what I would do. But the more I go, the less I know: will the fire still burn on my return? To keep the path lit on the only road I know, honey, all I know to do is go . . ." -- Indigo Girls
"And if there is a way to find you, I will find you; but will you find me if Neil makes me a tree, an afro, a pharaoh? I can't go, you said so, but threads that are golden don't break easily . . ." -- Tori Amos