And readers simply want to *read* about "no one else" but the main characters. And I can understand the sentiment, really I can -- because the point of fanfic is that one is using someone else's characters and setting, so if one isn't going to use them, then why is one writing it?
You already given the other reason... using the *setting*. I think it would be a fantastic thing to do a fanzine where none of the stories use the main characters, or are perhaps, allowed a single main-character appearence. Thus you could get plots like, "the Federation tries to take over a matriarchal planet; Blake, during his absence from the Liberator, arrives there to try to organize the locals only to find they have their own ideas on how to manage things, thank you very much, but the poor little man seems so sincere they could perhaps let him travel with the fighting squad as a mascot" or "Governor Le Grande discovers a horrible secret about the Federation which makes her determined to oppose it" or "Pollux, the interesting product of a genetic experiment by the Clonemasters, can change his shape at will. He is commissioned to morph himself into the likeness of Jenna Stannis to make contact with Avon and get the Liberator. However, once he dodges those watching him, he duplicates Servalan's likeness to gather resources for his own bid for personal freedom... Will he be able to lose his trackers and become a free and independent being?" All of these would be making use of some of the 'givens' of the Blake's 7 universe: Blake's fame as a rebel, and how it might not matter much on an outside world; filling in some unknown background on Le Grand; the technology of the Clonemasters and the weaknesses of the Federation as an organization. They would not need much in the way of the main characters, but could be fascinating ways of rounding out the Blake's 7 universe. Some people wouldn't read it because it would mean no revelling in the character relationships, but one could create *new* character dynamics between the cameo character and the original creations. The biggest challenge for the writers would be, I think, keeping the 'tone' of the universe. That is, Pollux would probably die; possibly in an ironic twist, shot by Dayna who thought he really was Servalan. Le Grand may be succesful in creating changes within her section of organization, but the story should hold foreshadowings that this is unlikely to last after Voices From The Past. One reason I don't read a lot of fanfic is that, from what I read about it, it sounds like the same things are rehashed endlessly. I like the idea of developing not just the characters but the universe. Why not write stories like that as original fics since the characters are mostly new or little developed in the series? Because 1. It's hard to get original fiction published. 2. You'd have to estblish all the givens; B is a big-name hero; F is evil institution and uses genocide and drugs to take over new worlds and quell posessed ones; C have the only advanced genetic technology accessable to the F and S wants to use it to get the L through A. 3. Once you had done all that to create an 'original' universe, some Blake's 7 fan will take a look and say, "It's an avatar universe."