I'm going to toss this out there (and hopefully not get punched in the face): I don't think a Spike television show or movie would be... interesting.
I love Spike, he's one of my favorite characters and his involvement helped bring both Buffy and Angel up a notch. But to me, he's just not main character material. Buffy was about a girl who was burdened by duty, obligation, responsibility. She had to save the world and still pencil in enough time to do her homework. The nature of high school, the capriciousness of life, ect. Angel was about redemption: how the most morally bankrupt man in history turned into a beacon of sacrifice and compassion, and how the fight never ends.
What does Spike have to offer his own show? He's not a Slayer; he's not bound by duty or obligation. My fear is that they would simply turn him into a blonde, sarcastic version of Angel. That's not good enough. The whole redemption and sacrifice thing? That's Angel's shtick, and Spike's simply nowhere near as good or as convincing as Angel is in that department.
What would be his motivation? His story? It's unfair to both Angel and to Spike to turn one of the most interesting and beloved characters into a pale imitation of the only vampire more important to the story than he is.
Spike is a great foil, a great supporting character, like Wesley or Gunn.
Just my two cents.