Snupin-type couples: Leon/D from Pet Shop of Horrors
I see the Snape/Lupin relationship as enemies or rivals. (Who later become reluctant allies and eventually lovers.) Which in hindsight made me realize that my first fandom OTP (Leon/D from Pet Shop of Horrors) has a similar theme.
Here's a bit of basic info about the story: Leon is a Los Angeles homicide detective investigating a series of mysterious deaths that seem to be linked to a pet shop in Chinatown that is run by Count D, an androgynously beautiful young Chinese man who claims to be running the shop for his grandfather, who is traveling abroad. The reader knows, although Leon does not, that in addition to the normal puppies and kittens and birds, D sells very special and magical pets to certain customers--for example, a mermaid that appears to be the reincarnation of a young man's drowned fiancee. However, he makes the customers sign contracts listing very specific instructions on how they are to care for these "pets," and if these conditions are broken, the result is often fatal. Leon can never make any criminal charges stick against D, but that doesn't stop him from trying, and so he spends a lot of time hanging out at the shop alternately interrogating D and trying to coax information out of him with expensive sweets and pastries. (D has a huge sweet tooth, and part of the fun is watching him go into orgasmic ecstasies over the fancy pastries that Leon brings him.)
They definitely start off as adversaries--a cop pursuing a suspected murderer, which is a rough equivalent to Lupin being an Order member and Snape being a Death Eater. There's some physical resemblance, too: D (the guy in my icon) has black hair cut in a chin-length bob similar to Alan Rickman's hairstyle in the movies. In fact, I remember someone on one of the Petshop Yahoo Groups joking that D and Snape could be cousins, which would make an awesome crossover fic! Leon is fair-haired, though his hair is blond instead of brown.
Their personalities are rather Slytherin and Gryffindor as well. D is sly and cunning, although I'm not sure that I would call him ambitious. He doesn't crave wealth or power, and his only ambitions seem to be to protect animals, to collect rare and unusual animals (usually magical or mythical creatures) for his shop, and to punish the people who hurt and/or kill animals. However, the Count does have a number of wealthy and/or powerful customers, which helps him stay out of trouble with the law. (Well, that and the fact that most ordinary humans don't believe that the creatures in his shop exist, which come to think of it, reminds me of how most Muggles are ignorant about the wizarding world.)
Leon is definitely a Gryffindor type. He's brave (often to the point of being reckless) and has a strong sense of justice. He also has a somewhat Gryffindor-ish habit of flouting authority: he continues to pursue his investigation of D on his own time even after his superiors tell him to back off. Leon is brash, outspoken, and often rude--which are not necessarily traits common to all Gryffindors, but it does remind me of the Marauders, although this fits James and Sirius's personalities more than Lupin's. Leon is hotheaded and loses his temper easily, unlike the more calm and serious Remus.
Count D is the cool and collected one--he is almost always unflappable and inscrutable. Leon seems to be the only one who can make him lose his temper, which probably helped sell me on the pairing since it's the people that we care about most who are able to get under our skin.
What's interesting is that although they start off as adversaries, their relationship gradually changes and grows into a peculiar friendship. As they spend more time together under the guise of Leon interrogating D with bribes of sweets, they seem to grow used to each other's company and even start to welcome it. After awhile, Leon's interrogations become almost perfunctory, more of an excuse for him to stop by and hang out at the shop with D after work. They save each other's lives on more than one occasion--or at least, Leon tries to save D, although D would probably have been all right without his help, since he's something more than human. But Leon doesn't know that, and it's the thought that counts, right? ;-) And there's a whole lot of UST: it's obvious that Leon (who is portrayed as being a very macho guy) is both attracted and repelled by D's effeminate beauty, and that D is drawn to Leon more than he wants to admit.
Similarly, Snape and Lupin are enemies (at least in Snape's mind) who become reluctant allies in the Order, and--at least in my fanon universe--eventually become friends and lovers.
It's not a perfect Snupin analogy; in some ways, the Snape and Lupin roles are reversed. In HP, Snape is the one who hates Lupin and is obsessed with finding out/exposing his secret, while Lupin never really considered Snape an enemy until the end of HBP. However, in PSOH, Leon is the one who hates D and wants to expose him as a criminal.
But still, I do think both pairs have a lot in common with the opposites-attract theme and the way that an adversarial relationship gradually evolves into something more. For some reason, that really appeals to me and is probably the reason why these are two of my top three OTPs.
TOKYOPOP published the 10-volume manga series as Pet Shop of Horrors (it's known as Petshop of Horrors in Japan), and sadly, it is out of print, but you might be able to find used copies at a decent price, or you might be able to borrow it from your local public library if they carry manga. There's a second series called Tokyo: Pet Shop of Horrors (Shin Petshop of Horrors in the original Japanese), but Leon makes only a couple of very brief cameo appearances in it. There was also a 4-episode Pet Shop of Horrors anime series, which still seems to be available on dvd on Amazon. The anime was great and is what got me interested in reading the manga, but most of the character development takes place in the manga, since the anime was too short to cover it in depth. Still, the animation is lovely and the Japanese voices are great (people who prefer dubs tell me that the American voice actors are good, too), and we do get some nice hints of Leon/D attraction.
And to tie it all together, I'd like to share a couple of PSOH/Snupin crossovers: