/be made to feel plausible, in the case of some types of AUs.
For example (I unfortunately don't share any of the fandoms you've mentioned, so I can't tailor examples to them), let's say I want to write a Harry Potter story where Harry and Draco, who mostly hated each other in school, become friends as adults. I could just write a friendship fic without explanation...or I could show (or briefly mention details here and there to build up a picture of) how the war changed Harry's perspective on Draco's level of badness a little and made the school enmity seem rather minor, and how Draco slowly came to realize that Harry was treating him much more civilly whenever they met and wasn't going around smugly bragging about defeating the Dark Lord, which, actually, was something Draco spent most of two years practically praying would happen...and they eventually wound up in a position to talk, and found a few more things in common, maybe apologized a bit or otherwise verbally put their school years behind them...and wound up in a tentative friendship that's growing stronger with the years.
That's a type of change that requires explanation, since it's the opposite of most of what's shown in canon, but not very much by way of an AU, since it's happening during a time we see very, very little of.
However, if I want to go full AU...
Let's say I want to write a fic where (and I'm cheating here a little, because I've done this to a certain extent already) I want to do a crossover fusion fic, in this case between Doctor Who/Torchwood and Harry Potter. I want to take a bunch of characters from DW/TW, and see how they'd fit into the Potterverse. What I have to pay attention to here is making them make sense as characters within the Potterverse without losing the essence of who they are in their original canon. Any big changes should be explainable, and the reading experience should feel, as much as possible, like we really are seeing these same characters as they would have been if they'd been born into a different world. Why? Because that's so much more satisfying than retaining a couple of the traits so the characters are more or less recognizable, while unnecessarily losing a lot more of who they are. I want the reader to feel like they've been given an AU twist, not that I've plopped somewhat similar characters down into an AU but lost the depth of the original characters.
(Parody and crackfic fall under a different category--at that point, just about anything goes, but it should be wild and/or funny. I'm sure someone else can better explain the line between goodfic and badfic in that category; I can identify it, but not really put my finger on it enough to explain well.)
Overall: the difference between goodfic and badfic often comes down to how well you can capture the original or how well you can explain or justify your changes. If you have one or both of those down, or mostly down, then, writing quality and story balance aside, you'll probably wind up with a goodfic. To get started, come up with your idea and start thinking about how it could come about!
~Z
PS: Bouncing ideas off other people can be really helpful in this process, and fun if you have someone who's really into it, though this isn't a must. Just a tip. Works especially well if you can workshop the idea together--i.e., if said person isn't afraid to suggest things or point out weaknesses that could be strengthened. They should absolutely not be putting you down or making you feel bad about your idea, but someone who semi-automatically joins you in adding a constructive criticism side to the excited idea planning session can be fantastic.
PPS: Welcome to the PPC! Funnily enough, this post probably exemplifies who I am: started off intending to just give the basic idea (hence 'on one foot'), added in detailed examples on a whim, and am now writing multiple post-scripts. Figures that'd wind up being my introduction. Anyway: I hope you have a good time here, and here, have a tiny, moving dragon figurine as a Welcome Gift!