Cristina, a girl in Lincoln's classroom whom he either has or had a crush on before "Making the Case", and who even got to - sort of - interact with a sister (Luna). It could have been interesting to know more about her and why Lincoln had a crush on her. However, not only does the whole "romantic" business go away like it never happened, but Cristina herself has all but vanished and it's unknown if she will appear again.note In "L is for Love," Lincoln has a new love interest, Paige.
The Loud brothers from "One of the Boys" left several fans disappointed, considering that they weren't given any actual characterization other than acting like Lynn (even the ones who are supposed to be the polar opposite of Lynn are just like her), as if to say all boys are boorish and aggressive.
The love interests introduced in "L is for Love", with the notable exception of Sam, Benny, and to a lesser extent, Chaz and Paige. As for everyone else, who knows if they will appear or even be referenced again.
Maggie from "Funny Business". Many fans wish that she would appear again and develop a friendship with Luan—seeing as how she enjoyed the latter's mime act at her birthday party plus Luan isn't depicted as having many friends—and of course see if it would evolve into something more. As of "Stage Plight", the latter is highly unlikely to happen. Even then, there is so much potential for Maggie to be Luan or Benny's other love interest, with a love triangle plot involving Maggie, Luan and Benny.
Rocky, Lucy's love interest for which she even tried to change her personality into "Back in Black", but that accepted her as she really was, and ended up being replaced without explanation, in "L is for Love",to the dissatisfaction of many fans. Perhaps the reason for the change was that Lucy and Rocky looked very much like a childish version of Mavis and Jonathan in Hotel Transylvania, as some people have noticed.
The girl with Lori's hair, Luan's teeth, Luna's musical vocation, and Lynn's taste for spicy foods, and her parents with Lincoln's white hair, in "Not a Loud." So many coincidences puzzled fans (to the point that some thought Lincoln's theory of being swapped at birth remained plausible, even after his parents' explanation) and left them curious about those people, with plenty of them wondering if they could be distant relatives of the Louds, and imagining how things would turn out if they eventually interacted with them. However, the mysterious family was forgotten after Mr. and Mrs. Loud presented their strange version of the story of Lincoln's birth. Additionally, The Loud House Movie fully debunks the "swapped at birth" scenario and renders the "distant relatives" scenario unlikely.
Girl Jordan. She was built up to be a close friend of Lincoln, but then she was shunted to the side by Stella to take that place. Nowadays, she mostly exists in the background, and only gets any dialogue occasionally. And she hasn't even joined Lincoln's gang (or, at least, not yet), which some fans say that she would make a great addition.
Carol Pingrey, whose potential as Lori's rival/competitor was under-utilized.
Many minor and One-Shot characters (such as Meli, Roxanne, the Bratty Kid who looks like Lincoln, Becky, the Yates family, etc.) have been phased out and haven't made any significant appearances since their respective debut episodes.
Lincoln's look-alike ancestor, the former Duke of Loch Loud, was also the only boy with ten sisters in a big family. Unlike Lincoln, however, he never had to worry about the everyday challenges and misadventures the latter goes through, nor was he ever in his sisters' shadow, and had lived an extremely luxurious life. It would've been interesting to explore the Duke's personality in depth and how similar/different he was from Lincoln, but he instead has very limited screentime, even as a ghost near the end of the movie.
Besides Lucille, we see very little of the Loud ancestors in general, besides one flashback and their ghostly forms at the end. Seeing how the characters' personality archetypes would play out differently in 1600 Scotland could have been funny.
have noticed.