Buffy/H.P. Lovecraft: Buffy may seem more suited to tackling slasher and monster movie cliches and turning them on their respective heads but I see no real reason why she can't also hunt down and dismember the eldritch horrors of the Cthulhu-verse and cheat both death and the madness that typically plague Lovecraft's protagonists. Plus, there's already a proven market for little ironic pokes and prods at Lovecraft. Surely you geeks out there are familiar with those "Hello Cthulhu" plushies, no? And I know for a fact that there're at least two H.P. Lovecraft/P.G. Wodehouse crossovers, so clearly Lovecraft's estate have a sense of humour about the whole thing. Could a Buffy the Vampire Slayer crossover not be far behind? There've already been hints at a Lovecraftian mythology what with the whole the-world-was-once-overrun-by-giant-demon-monstrosities thing, and that beast under the Hellmouth didn't look too unfamiliar. Frankly, I'd be surprised if some very literary Buffy geek hasn't already written this one and I'm just currently unaware of it.
Buffy/William Shakespeare: It's no secret that Joss is a big fan of the bard and famously had the BtVS cast round for regular play readings. I'm almost surprised that he hasn't already referenced the guy directly by shoving Titania or MacDuff's ghost into the precedings (ooh, now there's a way that they could exploit that Scottish castle in the comics...by bringing in the "Scottish play!"). A lot of Buffy episodes even come across like Shakespearean comedies half of the time, with the love polygons, mistaken identities and magical spells of "Bewitched, Bothered and Bewildered" et al seemingly coming straight out of Twelth Night or A Midsummer Night's Dream. What with the fairies, witches and magical potions already present in Shakespeare, a crossover like this wouldn't be too much of a stretch at all. Hey, even Doctor Who has had a Shakespeare crossover or two, you know.
Buffy/Frankenstein: Yes, we've already had Dracula turn up (after all, as Joss pointed out, he is public domain now, you know) and yes, there's already an episode or two in the Buffycanon that owe a debt of gratitue to Mary Shelley's monstrous classic for their inspiration, but still, just try and tell me that you wouldn't be excited at the prospect of Frankenstein's monster actually showing up. Frankenstein's monster was last seen wandering around in the arctic, right? Ok, so imagine the scene...Buffy and pals go North after some vamps who are going in for a sort of 30 Days of Night deal and exploiting the months-long nights that regularly affect human settlements within the Arctic circle. Buffy at first finds herself a little out of her depth, but don't worry, because soon supernatural help arrives in the form of - drum roll - Frankenstein's monster! Since he's already a character in the Buffy universe, maybe Dracula could even show up again, giving us a little taste of the old Universal horror flicks that would ofte shove these characters together.
Buffy/Nancy Drew: Buffy owes a lot to the 'teens solving mysteries' genre, and wow, would I love to see Buffy pastiching the whitebread world that Ms. Drew and in particular the Hardy boys seem to inhabit, maybe even introducing a little much-needed supernatural horror (at which point they truly would become the Scooby Gang, I guess). Nancy Drew fans would likely have kittens, but I still think it'd be a laugh, no?
Buffy/The Brothers Grimm: I've already briefly mentioned that series that Marvel Comics did a little while back in which they had their characters appear in adaptations of various world fairytales, and I still think that it's the kind of thing that the Buffyverse easily lends itself too. Just imagine, Buffy, Angel et al as postmodern heroines & heroes, princesses and princes in a series of feminist fairytale reimaginings that, in true pre-Disney fairytale tradition, don't for a moment skip over the bloody supernatural violence inherent to a lot of tales. Both Hansel & Gretel and Little Red Riding Hood have had their mentions in the TV series, so there's clearly an undercurrent of fairytale sensibilities, I'd just love to see it get truly exploited some day.
Ok, wow, I really didn't think that I'd think on this anywhere near as much as I have. Still, any more ideas for literary crossovers that you can think of? There's a couple of blindingly obvious contemporary literary series that I haven't mentioned, but feel free to bring up them and any others in the comments.