Dannyalcatraz,
By spell assassin, I mean like a killer for higher...but someone who wields arcane magic, rather than the traditional cloak-and-dagger type. In D&D 3.5 especially it seems like if you really wanted to get rid of someone, you'd be better off getting a level 20 wizard to scry, teleport, and disintegrate, than give the task to a level 20 rogue who'll have to waste a lot of time finding the target and generate a good opportunity to pull it off. I always thought a cool campaign idea would be a group full of wizards who decided to create a mafia-like organization.
CoatRackOfDoom,
I actually ran a campaign where each of the PCs controlled someone else's familiar. Honestly, the interactions between the familiars was more fun than the campaign itself. Very silly and non-serious, but fun nonetheless.
Another fun run was when we all got drunk and ran a campaign where everyone plays int 6 goblins. =) Just something completely different from the usual "serious" games"