Danielle, for those who don't consider her The Scrappy. She actually stands as one of two notable examples on this page, as had the series continued, there were plans to have the character eventually be adopted the Fentons and become Danny's little sister; thus making this a case of fans who like Danielle wishing this development somehow happened sooner, or — considering the episode she debuted in featured Sam and Tucker tired of being sidekicks — that Danielle could have at least become a regular member of Team Phantom, which could have also developed Danny's character further as a big brother and a mentor.
Remember Dark Danny? Danny's evil counterpart who was more powerful than him, had a complete lack of any sense of good in him and, considering he exists outside of time now, no longer needs Danny alive so he can easily kill the ghost boy? The character is the other notable example, as there were plans to have the character return as the main villain in a Grand Finale plot that never happened. It would take over 15 years before the character made a return in the comic book continuation, A Glitch in Time.
Valerie almost completely stopped appearing outside of cameos midway through Season 2. Though she had one major role in Season 3, otherwise it seemed as though the writers didn't know what to do with her. Fans find it especially galling that, considering a misunderstanding regarding Danny Phantom is why she became a ghost hunter to begin with, we never see her properly react to the reveal of Danny's identity: she never appears in "Reality Trip", while "Phantom Planet" has her in the background giving the same generic clapping and smiling reaction as everyone else in the crowd.
Several new villains like Undergrowth and Nocturn had interesting looks but were botched by typical "rule the world" plots and more Danny and Sam shipping. To be fair, Undergrowth had an excuse.
Bullet had a cool design and would've made a good Foil and dragon to Walker. However, he only worked with him in one of his episodes and barely got any attention.
The final episode featured a team of ghost hunters hired by Vlad called "The Masters Blasters," giving Danny a hard time and making people love them instead of him. The idea of Vlad having a team of ghost hunters working for him might have made them a decent Quirky Miniboss Squad and rivals to Danny...if they didn't appear in the very last episode.
Freakshow is not just the only human villain in the series outside the Guys in White (who he's also shown to be at odds against), but is also the only villain to use magical items as opposed to technology or ghostly powers. This, combined with having "ghost envy" (which makes for an interesting foil to Jazz, something the show does acknowledge), has many wish he was a more recurring threat. However, he's only used twice in the entire series.
Alicia is Maddie's sister and the only known member of the Fentons' extended family. Yet, she only appears in "Prisoners of Love" and has not made any more appearances in the series. Somewhat frustrating since we have yet to see her exact relationship with Danny and Jazz, too.
Kwan. A few episodes showed that he's a Nice Guy when Dash isn't around to influence him, and he even befriended Sam and Tucker after getting kicked out of his clique in "Lucky in Love." By the end of the episode, Status Quo Is God kicks in, and Kwan goes back to being a Jerk Jock. It would've been interesting to see him be the Token Good Teammate of the popular kids and even be an occasional acquaintance of Danny's group.
Harriet Chin. An old college classmate of Jack, Maddie, and Vlad, and was introduced in Vlad's introductory episode as a reporter who finds out that ghosts are real and is fired for being a "crackpot." Outside of a cameo appearance, she never shows up again.
