Well, what about pitching the potential of the combination of back-stories, new plots, and an established fan-base? I know this might be stretching it, but what about a 2-hour pilot to set the whole thing up? How do 2-hour pilots get made?
Well I don't know about all pilots, but I can and will use Buffy as an example, as I'm pretty knowledge on some stuff in that area. If my facts are correct (sometimes they are, sometimes they get confuzzled lol) then what happened was that Joss Whedon made the movie (puke puke puke) which was awful--he blames it on the director. I believe that when he wanted it to be reimaged as a TV show that either he went to the exec at FOX who ran the new entertainment division (whatever it's called) and said hey, I have this idea. I also remember something about maybe that exec, who was a woman but I don't remember her name, came up with the idea of remaking it as a TV show. Whatever...
What happened was that the script was written and it was shopped around to all the nets, CBS, NBC, ABC, FOX, and when none of them bit, they took it to WB. The WB was new then, and looking for a fanbase, so I believe they were more willing to take risks back then. They liked the script and and the pitch (obviously) and ordered the Pilot to be made. Now, the finished pilot that aired on TV for people to watch was 2 hours long. But, what was originally produced for both Buffy and Angel (when it hit the air) was a half hour pilot to just present to the network execs. This is just a rough copy of the pilot--they basically took 2 hours of material and cut all the stuff that was absolutely essential and then made it into a half hour. Casting and all that happens, and they shoot it. Then it was presented to the execs, who at this stage either love it or hate it, and order more or say hell no. They liked it, gave the Jossman some notes, and they recast a couple people, and went into full production on the real Pilot, which viewers eventually saw.
I'm not saying that this happens with all shows, but that is what happened with that show. And I can infer that the same thing probably happened with DS except the pilot produced was an hour instead of a half hour.