Television

Not going past Serenity makes sense, there's probably still some interesting stuff to do with the world, but it would just mess too much with the ending of Serenity. If the alliance is still around it should be extremely weakened, if it is still as strong and still hunting the crew of the Serenity then Wash and Book died for nothing. I don't think that's what the fans want or need to see. If they don't want to mess with that there are only two options. They could do a prequel to the series, basically the entire crew except for Simon, River and Book. The other option is somewhere between the series and the film, basically do what the original series was supposed to do. And they could pick up some things from the film, for example they could go into why Book and Inara left the crew. And I think the intro part of Serenity was always a little unclear, to me it seemed like River was captured and they freed here, others say its just a retcon to make it clearer why the alliance is hunting them to people who haven't watched the series, they could clear that part up.