The resolution episode for every cancelled show is simple:
All central characters are shown waking up in bed next to Emily Hartman, saying I just had had the strangest dream.
Fine, pass your law.
Who the hell is going to pay for producing your resolution episode?
Not really. By the time the cliffhanger airs, the network has already decided to cancel the show. It’s very rare for a network to respond to letter writers and renew the show and probably hasn’t happened since the 60s.
Low rated shows have been renewed (ironically, considering the myths, by Fox). Arrested Development had poor ratings after its first season, but Fox renewed it because they didn’t have anything better to replace it with. When it won the Emmy, that got it a third season. Dollhouse was the lowest rated network show ever to be renewed, but, again, Fox had nothing better, and thought the cult status might help.
Now, with additional outlets like Netflix and Amazon and Hulu, it’s possible that the letters might affect decisions, but it’s more likely to be demographics.
This one and “Twin Peaks.”
“How’s Annie? How’s Annie?”
Meh… some shows may not survive as TV but might survive as other forms of media like, mentioned upthread, films or comics or books. The thing is, though, it’s quite possible that it was canceled without the resolution to the cliffhanger being fully resolved. Hell, I’ve seen some films and shows pick up a cliffhanger and basically ditch it as quickly as possible, either because they realized it put them in a bad spot or they came up with something better.
But really, does it matter? If there’s no official canon to resolve it, I don’t really see how that’s necessarily unsatisfying. Let the fan community figure it out. There’s TONS of stuff even in fully finished stories that they don’t officially resolve. Maybe it’s a theory about a character’s mysterious origin or some underlying conspiracy or whatever. Why not just let the fan community figure out a satisfying conclusion and pick it up from there? Sure, a lot of fan stuff is garbage, but for the franchises I follow, I’m always fascinated to see some really cool theories about unexplained things or continuations of the stories, and sometimes it’s even better than what the original writers had in mind.
Hell, if there’s enough of a following for the fan community that can actually encourage someone to pick it back up and do something with it. We’ve seen this happen with a few shows recently, but that’s mostly through things like Kickstarter. The problem is, as those demonstrated, there’s a small number of diehard fans willing to donate a bunch of money, but they often don’t actually get what they needed to have been financially successful.
Either way, I sure as hell don’t want writers who are upset about losing their jobs bringing a story I love to a satisfying conclusion, I definitely wouldn’t want them to be forced to do so. And if they are interested enough to want to contribute to the fan community unpaid, they can just share their ideas about what was being planned or whatever afterward anyway.