OTOH, personally, part of the attraction to writing in a fandom (as opposed to watch or read only) is trying to fix some of the problems in it

I think fandom has gotten so used to the fix-it philosophy that we sometimes forget it doesn't have to be that way. My first interwebs fandom was X-Files, and that source text was, at the time, exceedingly good and not too skeevy.

Instead of issues to fix, it left a lot of blanks for us to fill in: the characters' personal lives were largely uncharted territory that fandom was free to explore. I read so many good and wildly different stories about what the hell happened in the Mulder family after Samantha was kidnapped, about the agents dealing with internal FBI procedures and politics, and of course a million variations on how they might hook up.

Once the show started to fill in fandom's favorite blanks-- more Mulder family stuff, more politics, Mulder/Scully romance-- the serious writers started drifting away, though that also might have been because the show itself started to suck.

I think any show that has an involving genre premise and good chemistry between a cast of interesting characters can become great fandom fodder so long as it leaves enough unexplored territory for fans to cultivate. It doesn't have to suck to work for us, it just has to leave us some room. Action/adventure oriented shows are great for fic fandom because they're plot-oriented, so the characters' downtime in the spaces between adventures is wide open for fandom exploration.

Contra X-Files, Burn Notice is an example of an action/adventure show that I don't think does/will attract much of a writing fandom... not because Burn Notice is unskeevy or too good for fixit fic, but just because the show doesn't leave enough blanks. Every week the main guy is shown taking on an adventure-- and also working on his overarching storyline (finding out who ruined his career as a spy), dealing with his on-again off-again girlfriend, and placating his pushy mother and ne'er-do-well brother. There's nowhere in the storyline for fandom to colonize.

So, yeah, sorry, this is long. My point is, fic fandom flourishes in the gaps that canon leaves, and so a lot of our source material ends up being crappy shows because crappy shows tend to leave lots of gaps: they neglect characters' backstories, they don't account for how the characters live their day to day lives, etc.

But just because those kinds of gaps are most often found in crappy shows doesn't mean we always have to settle for crappy shows. If we know what we're looking for, maybe we can find what we need in good shows, too.