Actually, I think Roger is the closest thing Stan has as a best friend, since he sometimes helps him out. They do do have a friendship with each other(remember Roger Codger), even though Roger ticks off Stan vice-versa.
My thoughts on a fourth Seth show aren't really positive. The Simpsons leaving Fox Sunday nights will never happen since the series is still profitable, so American Dad might get the boot (which is a shame because I would've wanted one more season after Season 6.) Also, it's another family show but with a female lead :shrug:. At this point, Seth should be tired of doing family shows and go for something different like Matt Groening with Futurama.