For me the Charlie thing would be terrible. I don't dislike her, but I would not want an entire series based around her. There are plenty of other routes to go with this show when it comes to a spin off. It sucks Bloodlines was not picked up, but the network has still said they are open to other spin off's in the future, which means we'll probably at least see one more back door pilot next season.
They have plenty to work with. They could go the route of doing a show that focuses on Samuel Colt or something. Or a show that focuses on a young John Winchester hunting monsters and slowly piecing together what Azazel was trying to do with Sam, etc. They could go the magic route, there are a variety of magical characters that could prove interesting to have a series involving them or something. Patrick the witch comes to mind, haven't seen him in a long time.
It's also possible for them to do a Men of Letters type thing without Sam and Dean..what if Sam and Dean die in the series finale or something? Or it otherwise shows them passing on their fortress, etc. to the next generation of hunters. There is a whole lot with men of letters they could do, they wouldn't specifically need Sam and Dean around. Especially since they really are not like the Men of Letters exactly. They are more hunters then anything else. Maybe Sam might of fit with them, but Dean? I honestly wouldn't of seen him fitting in with the group..or at least, as we saw them portrayed back in the 1950's, etc.