Buffy/Xander: I actually find this one strangely easy to accept in a theoretical way, in canon. I honestly feel like the show could have diverged into a B/X relationship at almost any time--even while Xander was with Anya--if they'd wanted to pull the classic plot rabbit of romance from a hat and have Buffy suddenly look at Xander in a new light. Because he of course would be all over her like in three seconds like white on rice. The question is, what would lay the groundwork for Buffy's side of things; what would make her look at him differently? The first thing I think of is the phenom where a guy starts to look shinier when a friend of yours--or a rival--develops a crush on him. But with a few exceptions Buffy tends to have a peerless indifference to other people's love interests. She didn't reconsider Xander as potential man-candy when Cordelia began dating him, for instance, and until Faith came along, Cordy was the closest thing Buffy had to a peer. And when Buffy finds out that *Faith* had sex with Xander, she pretty much writes him off as a "big joke" (a big joke to Faith, is what she actually says, but it doesn't indicate that her own opinion of Xander has been elevated by learning about their liaison).
So, ruling out the covet-thy-neighbor's-boyfriend scenario, the next one that occurs to me is hurt/comfort where Buffy's opinion of men has reached its nadir--she's hurt, she's been burned--and she turns to Xander as friend and confidante, for one of those cozy popcorn-on-the-couch talks, and she cries, and he holds her and doesn't even think to take advantage of the situation but instead says something so profoundly tender and selfless and gentle and Buffy-loving that she just stares at him in wonder; and here it's his obliviousness that saves him, because for dramatic purposes the tables have to be turned. Xander can't realize that she's finally reciprocating his feelings. Not right away. He has to be the object of her female gaze for a while, and I'm thinking that he'd have to be in a success zone, like the Alpha Xander of "The Replacement." Great job, great apartment, and maybe he's made some declaration of independence where he's like, "I've decided I can't just toddle around after you every night, getting in your way. I've got to do my own thing." And Joss et al would have to give him his own B-plot storylines for a while: Xander helps out some guy at work who's plagued by demons, Xander rescues a damsel in distress. And Buffy keeps getting these unanticipated testimonies from people as to Xander's manly, good-hearted competence, and she finally gears up and makes a move, and the scene plays like this: after some clumsy wooing and circumlocutions, she launches into this long speech where she says she knows he's always been there for her during her tragic romantic history and she's always thought of him like a brother and you of course don't want to date your brother, but he's not a brother, and she knows that, because when she looks at him now she sees a guy, a really amazing guy who's kind and funny and loyal, but not like a dog! (she says hastily), not at all dog or brother-like, and she knows it's probably too late, because she's turned him down in the past, and he's got his own life now, and some nice girl is interested in him and she doesn't even appear to be a demon--and Buffy's winding down into an unhappy ending to her speech because he's just looking at her gravely and saying nothing, and instead of convincing him to go out with her, she's turned it around and told him all the reasons why he wouldn't. "And you're not really interested, are you?" she ends on a resigned, rueful note, looking down and already starting to draw in on herself because she's made a fool of herself, and he says, "I'd have to be crazy, Buffy," and he steps closer, "to *ever* turn you down." And she's all pansy-eyed and uplifted and hopeful and they kiss and then it gets surprisingly sexy and all across America thousands of fanboys cheer and do that little Arsenio hand-pump.
But of course they'll only last a season or so before breaking up and settling back into friendship because the lesson behind all this is cookie dough is dangerous to your health. Sure, it tastes good, but there's raw egg in that, man. Bake it first for three years at 400 degrees. And then--enjoy!