I still think when it comes to let’s say necessitating Kitty Pryde to be an anti-heroine for good or Barry Allen being Dante Alighieri had they been fanboy/fangirl inserts this would be the times when they become their own characters. Albeit going further away from their fannish origins in a way fans won’t immediately recognise. Even if that does make them seem less Mary Sueish.
I actually think in Kitty’s case, the real issue lies with not letting her go where she’d logically become. I even have a feeling that Kitty Pryde should’ve become the character Psylocke ended up as in that the former didn’t start out as a psychic ninja at all whilst the former had training even if it both happened against their will (Kitty through brainwashing, Betsy through body exchange).
Kitty Pryde becoming what Psylocke ended up as would’ve done way more favours and the best way to keep her badass whilst not making her Mary Sue. It would be the least Mary Sue she’s been ever since she was an assassin in Age of Apocalypse. But that necessitates having to do away the more ridiculous aspects and stick to where she’d naturally go and become.
(I still think it would’ve been better for Kitty Pryde to be a deadly ninja given Psylocke reverted to her old white self, as if the latter wasn’t meant to be that character for long whilst the former easily could.)
I would actually be fine if Kitty Pryde became an assassin full-time. Then again that necessitates following the more logical, organic path than what we’re usually shown. As for Barry Allen standing in for Dante Alighieri or Red Riding Hood should Caitlin Snow become a werewolf, the real issue isn’t that he lacks a personality in general. But rather he lacks a personality beyond being a fanboy surrogate.
Should Caitlin ever become a werewolf, that necesitates Barry to be Red Riding Hood. Oddly enough that would be much more relatable to a lot of people because they’ve heard of Red Riding Hood first. It’s basically when a child leaves home to greet their grandparent or relative but it doesn’t go as expected. Either they survive at will by conning their captor, survive by being rescued by others or die anyways.
(Barry would most likely escape from her in advance.)
Likewise if Barry Allen were Dante Alighieri and Caitlin Snow as a werewolf, it would be too easy especially for those familiar with both the writer and his work Divine Comedy. Perhaps a properly contemporary version of it complete with Pied Piper, Heatwave, Captain Cold, Weather Wizard and even Zatanna in hell. But that would mean Barry Allen would’ve outgrown the fanboy surrogate role either way.
Rather than being an idealised fanboy surrogate, he’s either a properly adult version of the Red Riding Hood (because he’s one himself) or a truly modernised Dante Alighieri in a way Dante Sparda and the like aren’t. On one hand, that does make Barry Allen into a proper update of a familiar folkloric or literary character. On the other hand, that would involve a much bigger change to the mythos.
Perhaps even drastically changing Barry’s personality in the sense of being no longer a glorified fanboy. The odd fact that both works predate the Flash should indicate that the Flash’s own trajectory lies not in Greek antiquity, but in medieval folklore and spirituality (weirdly enough, the Divine Comedy’s right up there with the been to hell testimonies and had Dante lived today, he’d be a Pentecostal).
A bigger change than say retconning his backstory as he actually becomes a proper update to either genre (as in different media). Maybe way too well but still.