I like fan films. I like 'em a lot. There's an earnestness to 'em and a flying-by-the-seat-of-your-pants DIY quality that is hard for me to ignore. It shines through the most awful acting, the most cardboard sets, the most poorly matted CGI. Have you noticed how similar a lot of them are, though? Of course, given their nature it's a kind of 'well, duh' thing to call them derivative. Even still, I find myself wondering exactly how original you can really get in these things before you lose your audience.
I was involved in three fanfilms. One was my own project, another was a Dr. Who thing, and another was a Bond parody. All three fell apart. It's the nature of these things to fall apart, particularly in the pre-internet days when no one would ever see 'em, and studios would sue you in a heartbeat for these kinds of shenanegins. Even now, I figure there are ten aborted fan film projects for every one that sees the light of virtual day.
My project was a Trek show, set on a more-or-less derelict experimental ship that was pressed into service unofficially to go about the federation and clean up other people's messes. It wasn't dark or oppressive or revisionist or anything, and it had the same basic 'planet of the week' structure (Or it would have, had we filmed it), it was just a little bit more cynical, but in a kind of fun way. The premise was that there were a lot of Captain Kirk types out there, and most weren't as lucky as Jimbo. Thus: you needed someone who could fix other people's mistakes in such a way that allowed the Federation a degree of plausible deniability.
When I told people "I want to do a Star Trek spinoff kinda' show original episode story dealie kinda' thing" (The term "Fan Film" not having been invented yet," I had no problem finding a crew and a cast who were pretty interested, but once I explained that the story would be about a different crew on a different ship, I lost a lot of 'em, and had to find more. The when I explained the premise, which differed from Trek *only* in terms of the mission, but not the format, I lost more, because they only wanted to do exactly the same thing they'd seen on TV all their lives, so I had to find more. Then, when we went with a different bridge design (It was cool!) and different uniforms (Because we couldn't afford ones from any of the shows) more of 'em walked, and I had to find more crew.
Ultimately, I just said "Screw it, let's just go the extra fifteen feet and make a wholly original film unrelated to Trek," and then the whole project fell apart (Despite it resulting in a better script since we were no longer attempting to shoehorn it into the trekiverse) because people only wanted to do Trek, and doing something new (Albeit clearly a derivative and a knockoff) just wasn't soemthing they were interested in. I'm not arrogant enough to say they were wrong.
If we're honest, Trek made the same series five times. There was only one significant departure, and it is *clearly* the exception to the rule and it wasn't all that popular at the time. The studio couldn't try to drag the franchise back into the box fast enough. Likewise, Stargate made the same show twice, varied the format once, varied it too far, and now the entire franchise is dead.
The fans like what they like and *in general* what they like is pretty much what they've already seen. That's not a bad thing: I've been wanting a fourth season of TOS my entire life, and Exeter gave me that, then Cawley did the same thing far more consistently. That's fantastic! I'm not complaining, and if people like it (And I like it, too), hey, that's the sort of thing people like, and he'd be a fool not to do more of it.
My personal problem - and this is just me talking about me, it in no way is meant to reflect other people's fan projects - is that I don't see the point in doing the same thing everyone else is doing, unless you've got something significant to bring to it that no one else has. Why bother to record someone else's song, unless you've made it your own in some way, right? Cawley has done that, the Farragut guys have done that, a few others as well, but more haven't. You figure you're putting enough money into one of these things to buy a car, and gosh only knows how much time and effort besides. If I'm personally going to put that much into it, I keep thinking, "Just take it a tiny bit further and turn it into your own completely original standalone piece."
That's my quandary: I want to tell my own story, but do I want to tell it in someone else's universe or create my own? There's advantages and disadvantages to both. For instance, I've got a Stargate idea that everyone tells me is great, but with just a few minor twists it becomes my own thing and, well, there you have it...
So the question becomes: Will the fans be willing to watch a fan film series that takes place entirely on a Romulan ship with no humans involved at all? Will Stargate fans watch a series about lawyers (Not my idea, just an example)? Will Babylon 5 fans watch a show set on Centauri Prime that never visits the station? How far afield to you want to go, and how far will fans follow you before they decide to head back home. That's an inherent limitation of the subculture, as I've seen from being inside it in the early days.