I'm up for changing the site rules if fandom characters are a nuisance, but just wanted to verify since there seems to be some confusion: do people want both canon characters and derivative characters banned, or just canon characters? Where these are defined as the following:
Personally, I'm not sure anything should be banned merely for being a "nuisance". Perhaps you're understating "serious problem in the community" with that word, perhaps I'm just picking at linguistics, but someone else's behaviour being a mere nuisance to someone else should not necessitate the banning of anything.
With that said, onto the specifics:
- Canon characters:
- EG: Re-uploading Naruto with no changes made to his design or history (usually includes GIFs and screenshots from the anime or copy pasted paragraphs from the character's wiki page)
Straight-up ban. From what I understand these should already be against site rules ("don't upload art/characters you don't have permission to use"), but I'm all for re-writing those rules to make it crystal clear that the usage of any art or characters is automatically disallowed without direct permission from the creator. That's just common sense, both legally and morally (why do I seem to be throwing that phrase around so much lately?)
Should typically be permitted except for creators who have explicitly disallowed derivative works. Derivation has always been the right of the consumer of any media since literally the beginning of fiction, and directly contributes to more art (even Shakespeare did it). Art is inherently derivative but if someone outright says "don't derive from my work", they should probably make their way onto a DNP list. This applies to huge creators like DC and small creators like LiLaiRa.
- explicit redesigns (eg. Naruto as a dog),
A grey area, where referring to the creator's statement on derivation may be wise. I think there's a line here between "literally someone else's design tweaked as minimally as possible to fit under the derivation rules" and "a design inspired by someone else's work, clearly permissible under derivation rules". That sort of line is hard to draw and I leave it up to staff, but there is a line and I think it needs drawing.
- characters that're described as an OC or sona but resemble the canon character in both personality and appearance or cosplay them 24/7
I think this fits into the same grey area as above.
- personalised playable MCs (eg. customised Kamui/Robin, Frisk, Gudako/Gudao),
EDIT: This has been explained to me as customisable characters from games (mostly MMOs). Fits under "characters belonging to a canon species" for the most part, but for certain games, such as Mass Effect, the "customisation" is superficial and all possible interperetations of the character are equally canonical, pushing them closer to the "Naruto as a dog" section.
- characters belonging to a canon species (eg. pokemon OCs, LOZ OCs),
Should be permitted unless the creator has explicitly disallowed them. Pokémon being the obvious example, there are dozens of characters belonging to a single species, all with clearly distinct personalities. In many cases, design changes are minimal or even nonexistent. Ash's Pikachu, for example, is the generic Pikachu design. There's nothing special about the way it looks, its distinguishing feature is its overpowered Electric attacks. You can't see that just by looking at it.
- characters belonging to a canon setting (eg. BNHA/Hogwarts OCs that use the school uniform taken from canon designs), etc.
The most classic example of derivative works in art, once again these should be permitted unless the creator says otherwise (and since JKRowling, at least, outright approves of derivation, Harry Potter fan-characters are very safe).
- This is quite a wide range, so if you have specific thoughts on what you find unacceptable or acceptable for a derivative character that'd also help with gauging the community's opinion for the new ruleset.
Regardless of whether it's a fan-character, derived from canon content; regarldess of whether or not the character happens, by mere coincidence or by the facts of the canon (eg, Pikachu, as above): the uploading of literal "canon art", that is the uploading of art not created by the uploader, and which the uploader does not have express permission to use, should be blanket-banned if it isn't already.
Again there's a grey area here. Memes and image macros being the obvious example, but they slip neatly into the "public domain" and "share/share-alike" laws by the nature of how they're created and shared, so they should be safe as long as the rules are written clearly.