Let's Just be Friends
Most writers of Angel and Spike would agree, they just look pretty together. These two vampires are so easily slashable that it is easy to forget they have never had an openly gay relationship in canon, furtive looks and name-calling aside. Spike and Angel, as far as Whedonverse canon are considered, are straight. They enjoy sex with women.
I know, it's shocking.
Why is it then that they are two of the most frequently slashed characters on BtVS and Ats? An even better question is this: Is it necessary to slash them to make a good story? Of course not. We've all read a piece of fiction at some point or another that was so good and plot oriented we forgot to look for the sex. Maybe you want to explore this side of writing, or maybe you have never been comfortable with slash but love the characters of Spike and Angel and want to write the pairing. A good author can turn a plot bunny into a gripping tale effortlessly, for the rest of us it takes work, research and work.
It also helps to keep a few of things in mind when writing non-slash, or even completely non-sexual Spike/Angel fanfiction...
Angel and Spike have not only had relationships with women, they have had relationships with the same women, Buffy and Drusilla. Remembering this point can lead to great stories. Comparisons of the two women, their reactions to their different vampire lovers, the jealousy and angst Spike and Angel have visibly shown on the shows. Think Spike's reactions during Season Two's Becoming and Season Seven's End of Days, he does not like Angel touching his women.
Angel and Spike have reluctantly worked together, in Lover's Walk, and made their scenes together interesting on many emotional levels. They have an inherent distrust of each other and are constantly trying to one-up each other. That emotional banter makes for great conversation in fanfiction, the one part of the Whedonverse that has all the time in the world to explore the themes the show cannot properly delve into.
Spike has made a habit of torturing Angel. In What's My Line II, In the Dark, and to a lesser extent Becoming II, Spike reveled in beating Angel, watching him be beaten, or just hiring someone to do it while he was gone, content in the knowledge that Angel was being tortured even if Spike wasn't there to watch. This sadistic twist in their Yoda/Sire/Childe relationship brings up many questions of payback, is it due or worth it to Angel to pursue?
Spike and Angel also both have souls now. The questions Spike must have for Angel are complex. How long does it hurt? Is there such a thing as redemption? When coupled with the possibility that one or both of them may someday be made human again the possibilities expand ten-fold. How would the dynamic change if one of them were no longer a vampire? All of these questions make for interesting fiction.
All in all the questions posed above can be pondered, and possibly answered, in slash or non-slash stories. While PWP and smut are always fun, there are so many other stories to be told, and now that Buffy has ended and Angel is on summer-hiatus writers have the whole summer to write before getting Jossed in the fall. Hopefully I have given some useful ideas and points to ponder during the interim.