Respecting the canon may be critical element, but writer skill makes all sins forgivable. To illustrate such, I recall a titanic crossover fic series called The Crystal Tokyo Chronicles (I think). Anime, and lots of it. Sailor Moon, Tenchi Muyo, Ranma 1/2, Patlabor, Dirty Pair, Bubblegum Crisis -probably more I can't recall. The whole mess is set in a mega-utopia founded by the sailor scouts called Crystal Tokyo set in 2900 or so.
Sparing the blissfully ignorant all the details of the original material, let's run with the assumption that having the Scouts all be insane in one way or another, the Ranma cast that's alive be immortal from Tao Supremacy, and time traveling clones of the Knight Sabers doesn't really bear much semblance to the canon material.
What makes this mess work is the quality. The Sailor Scouts are mad because they're eternal little girls utterly unprepared for the scope of the stress and tragedy they'd encounter, and then be forced to repeat over and over those same circumstances and mistakes because death is the only mercy destiny offers. Ultimately having nigh-godlike power is no kind of gift to the well-intending. Entropy stops being funny about the fifth or sixth time you see your life's work collapse again.
Ranma and Co. are the most dangerous things walking, but unable to much more then weather the losses of everyone and everything -live long enough and you'll see everyone die to terrorism or freak mechanical failures or deadly ennui. Incidentally they've got the easy route; so many of the eternal rival characters like Ryouga had to make deals with devils. Ryouga himself ate the flesh of a mermaid and gambled on immortality that way (a nod to another of Rumiko Takahashi's works the Mermaid series). Another became a vampire because the same "I'll be with you no matter what" drive that defined her in the base series required a brutal sacrifice in the end.
The Knight Sabers -the survivors that is, are pretty much ok. Ironically Priss and Sylia are able to function because day-to-day survival is how they lived before and neither is actually afraid of just dying one day. Life is hard and unfair, live and love while you can.
So! It's hyper canon-defiant, utterly fan service loaded, and completely excellent. What makes Crystal Tokyo shine is it's willingness to exceed the core material. Truth be Ranma 1/2 and Sailor Moon are pretty much 1-trick chumps. Same plot, same rivals, same situations, same tropes and ultimately the same fatal repetition. I don't doubt there are Mary Sue characters in there, but if so they're subtle -or maybe not, I can't say I know every party involved perfectly.
Canon has it's limits. Being true to the core universe only gets you so far, especially when you have kicking about an iconic like Kirk -who never loses (hell, if I was asked to define Star Trek tropes I might not get more then one or two). In the endgame quality is the final arbiter, and all the Strunk and White in every literature dean's desk doesn't spin into a fine yarn.