I was going to talk about Maypul, but Maypul discussion can wait for the moment. She is super good, and this will become clear enough in time if it isn't already.
The character I think we really need to talk about is Zetterburn. There's becoming a near universal consensus on Steam and anywhere else I look that he is a totally cancerous character to the metagame, generally because he's so braindead basic while still remaining just as effective as most of the other characters. Typical Zetterburn gameplay is "put out down b and stall with fireballs until around 50%, then attempt dash attack into Forward strong and repeat". This is beatable, but you have to work so much harder than the Zetterburn player does that it seems people find it intensely frustrating to play against. Among very strong players, I've noticed a kind of gentleman's agreement not to play Zetterburn, so I can't speak to just how good this strategy can be, but I think there's precious little argument to be made that he is not by far the most hated character and is probably hurting the community overall.
I've thought about it for a while, and I think the main problem with Zetterburn is the way Fire Amp works. Double Knockback in a game like this where recoveries are weak and blastzones are short is simply such an insane level of reward that there's no way it could not completely centralize his gameplay. It's the difference between killing at 60% and 120%, so you're functionally dealing 60% extra damage for landing that smash attack while they are on fire. This is simply ridiculous. Compare the level of reward other characters get for their unique mechanics. If Maypul marks you, she gets at most an extra Strong Attack, dealing maybe 15% extra damage. If Orcane gets you on a puddle, he gets a bit more range or to use his down B. If Kragg hits you with a rock, he gets like 10% damage. None of these mechanics are anywhere comparable to the boost Zetterburn gets from Fire Amp, they simply enhance those characters rather than completely define them.
I feel that Zetterburn needs a fairly significant revamp, and as part of that revamp, Fire Amp needs to be cut to being a 30% knockback boost rather than 100%. This is still quite significant, it could mean the difference between killing at 120% and killing at 90%, but it no longer forces this ridiculous dichotomy where he either kills you at insanely low percents when you're on fire, or he doesn't kill you at all if you're not. This way there would be a much better incentive to explore his combo game and all the rest of the things the character is capable of (and it's possible there may need to be some buffs in there, but right now his gameplay is so centralized around Empowered Strong that there's almost no way to tell).