I would hope for adult content, and the system proposed; the Amber rules. What character types did you have in mind?
*dusts herself off after one more midterm* OK, I'll disclose a bit of the plot since there are some people discussing time settings and character types.
Broadly speaking, any type of character will do, but bear in mind that very quickly after games get started, things go to hell. Literally.
Remember Glory, the Big Bad from season 5? Her backstory in a nutshell; a demon god that ruled over a hell dimension with two others in a triumverate back in the day. The two other hellgods feared Glory so much that they joined together to battle her; she was bound to mortal flesh, conceived of a plan to return home, etc. etc. etc. Buffy et al. manage to defeat Glory, at the cost of Buffy's life, who makes the ultimate sacrifice in order to seal the portal opened to Glory's dimension.
Here's where the noncanon starts. What if the other two Hellgods, sensing Glory's imminent return when the portal opened, flew into the mortal realm to fight Glory in a last ditch effort to stop her from returning home. What if, when Buffy sealed the gate, the two Hellgods were trapped on Earth in their full power and bad black naughtiness?
As a last aside, since discussion has been mixed on Amber system, and I came to realize how difficult it would make things if certain events occurred (e.g. becoming vampires/werewolves) I've decided to go systemless.