Basically. That's what the writers have to find: something that makes sense.
And there are AUs out there that I do enjoy. They just have to respect the canon for me.
To put it in a simpler way: I'm totally cool with the writer bending the rules...but I'm not cool with them breaking the rules. Bend, don't break.
If I were to give examples, by using potential story elements within an Undertale fanfic (since UT fics are on topic):
I'm cool with this: WD Gaster is Sans' father, or some kind of relative. I'm actually totally cool with this, and even support it concerning headcanon. Both are skeleton figures, both are related to the science realm within the UT game, and both have powers that are very unique. It makes sense that they know each other and/or have some special relation.
I'm not cool with this: Sans being made into an all-powerful god-like being, moreso than in UT. I love Sans, but if the "he actually had more powers that he was just hiding until now" story point came into play, I'd be more likely to have mixed feelings on it at BEST. It *could* be done right, but if one's favorite character is made into an indestructible Mary Sue, I'm not a fan of that. The depth of the character begins to die and they lack tangible weaknesses due to such circumstances. I'm not cool with this kind of plot point because it 1.) disrespects the canon (Sans is very secretly strong, but he has tangible weaknesses and isn't a god-like being in any way, and why would he want that in the first place; it's not in his personality), and 2.) makes the character less relatable.
Now, risky tropes like the S tier power level one I mentioned in the second paragraph could be pulled off into something positive, and there is gray area there...but something like that is rare to find and is normally pulled off poorly. That's why I'm wary of fanfics...because while canon CAN be respected, it can be hard to do when you let your creativity fly away without any boundaries.