I think what makes this one stand out as even better than the already really good base game is the way it taps into the same primal mystique that motivates us to put characters into games they shouldn't be (be it official bonus modes or fan mods) and works that into both the game design and the story. Plague Knight is an outcast, both in a narrative sense where they're shunned by the local villagefolk and forced to live in an underground lab and in a mechanical sense where it is abundantly clear that the main campaign's levels were not made with them in mind. Sometimes it's in the sense of unintentional side effects of a character with drastically different attacks, movement physics, etc. like many such cases in Propeller Knight's stage and other times it's parts of levels hard coded to not work like they did in Shovel Knight's campaign such as the flying bushes in Specter Knight's stage and the rainbow bridges in Polar Knight's stage. However, while this might seem like something that only works to Plague Knight's detriment, it also leans the other way around. Plague Knight's sheer quantity of movement options allow them to cheese many platforming challenges that would have given the cerulean coward trouble and the increased range and spammability of their attacks makes cheesing bosses even easier. Controlling Plague Knight feels like controlling a character modded into a game they very much weren't supposed to be (ex. that Sonic Generations mod from a while back that allows you to play as Mario 64 Mario, complete with their very specific movement physics) and also playing the role of the cartoon mad scientist character that Plague Knight is clearly riffing on; never having a chance of winning if you play fair and square but instead getting success through being a cheating bastard. The world wasn't made with Plague Knight in mind but goddamnit if they aren't going to hang on by any means necessary.
Also, Plague Knight and Mona are yuri to me.

I am Alucard. 200.6% map completion, 1890 rooms, too ez. Also, best metroidvania I've ever played, sorry Hollow Knight, amazing castle design and the movement is perfect. I love the gravity boots, imagine not being able to infinite super jump in a metroidvania (sorry Hollow Knight). SOUL STEAL. ALUCARD SHIELD + SHIELD ROD = WIN. WHAT IS A MAN? A MISERABLE LITTLE PILE OF SECRETS. BUT ENOUGH TALK... HAVE AT YOU! FOR WHAT PROFIT IS IT TO MAN IF HE GAINS THE WORLD, AND LOSES HIS OWN SOUL? MATTHEW 16:26 I BELIEVE.