There are those who are disappointed about Sprig and Maddie's engagement ending, as she seemed to truly like Sprig and they feel she's a far more interesting character than Ivy.
In the season 1 finale, "Reunion", it is revealed that Hop Pop's run for mayor of Wartwood led to frog rebellions across the valley, leading to the events of that episode. The rebellions, however, go completely unseen for the entirety of the show. They also end up not contributing to the main plot any further than the events of that episode, and Grime and Sasha's attempts to quell the rebellion simply result in the dissolution of their division of the Toad Army, and the frog rebellions in Frog Valley are mostly forgotten, only being alluded to afterwards at best.
Likewise, Grime and Sasha's toad rebellion is also mentioned by General Yunan in "The First Temple", but is only seen in "True Colors" attempting to invade Newtopia, only to serve as a quickly disposed-of Disc-One Final Boss that only serves as a major threat in the first half of the episode, being forgotten in the second half of the episode aside from the destruction of North Toad Tower.
Despite season 3A taking place on Earth, with a few characters commenting on Anne's disappearance, Sasha and Marcy's involvement and the ramifications of it is never even touched upon, with the two's parents not even appearing once. Furthermore, considering how Anne gave their families letters in "Froggy Little Christmas," one would expect at least a mention of it, yet the consequences of Anne doing this are pretty much ignored. At the very least, an episode or a scene featuring the Wu and Waybright families worrying about the other two would've made a lot of sense, since Anne's parents fretted over her when she came back.
"Toad To Redemption" set up Bog and his gang eventually returning as antagonists in a later episode, but they never reappear as an antagonist in any episode at all, at best making a background cameo in "Beginning of the End". Same goes for the grubhog, whose last major appearance set up a conflict or war, but also only returns in a background cameo in the same episode.
Ivy never interacts with her grandmother Sylvia, and Felicia, Sylvia's own daughter, interacts with her only once.
Felicia Sundew is given an interesting background of having been a world traveler in “Ivy On The Run”. This backstory of hers, along with the fact that she met Ivy's father sometime during her tea travels, is never brought up again, nor is Felicia given more than a couple of lines for the rest of the show.
Despite starting up a friendship in "Maddie & Marcy" (being Marcy's first friendship outside Anne and Sasha that didn't involve manipulation or superficial reasons), Marcy and Maddie's friendship is never brought up again afterward with Maddie never reacting to Marcy going missing in "Turning Point" or even finding out about Darcy as well as neither saying goodbye to one another individually in the series finale. Additionally, some thought that Maddie being able to raise the dead (albeit imperfectly and only shown for animals) should have played some role in resolving Marcy's apparent death at the end of season 2.
The subplot dealing with how people of Earth is handling the coming invasion of Andrias is also left untold.
There are fans who feel disappointed on Sasha and Marcy's minimal interaction with each other outside of Anne which could've provided good opportunities for both of their Character Development considering each others' own betrayals revealed in "True Colors".
Adding to the above, Sasha and Marcy's Calamity Powers were only fully shown in the Grand Finale, with their eyes only glowing once in Season 2 before the stones got charged. Unlike Anne, both were unable to unlock the full extent of their powers. Furthermore, Sasha using her powers or possibly having a Traumatic Superpower Awakening like Anne did in "True Colors" during her fight with Darcy in "All In" could've been an interesting scenario to see. Likewise, Marcy awakening her Calamity form while possessed by the Core would've been compelling as well.
There are fans who feel that the ending was extremely disappointing and that permanently separating the Calamity Trio from their found frog families, as opposed to two-way transportation, is too bleak an idea to end on. They feel that it is a waste of a perfectly good found family plot, and that found family doesn't typically break up under such circumstances. Most fans tend to focus on Sasha, Anne, and Marcy splitting up for several years, and it often flies under the radar as a criticism. Some fans who felt similarly about the finale even believe that alternatively it should've ended with at least one of the Calamity Trio being stuck in Amphibia permanently, believing it to be a more plausible ending given how the Gems were destroyed after The Core's defeat and would still be keeping the show's message about accepting change intact, with them still having their Amphibian found families in their lives to help make the drama more balanced.
Despite being built up as big parts of Sasha and Marcy's lives, Sasha and Marcy's parents are never seen onscreen at all. We don't even get to see their backstories about each other either.
In "Escape to Amphibia", Anne is planning to leave Earth to go back to Amphibia. Though it's worth noting that this takes months right after she came home after being gone for five months. This could have led to Anne being confronted by her parents, particularly her mom, for wanting to leave them after being missing for a long time or worse, even getting disowned by them, knowing how many mistakes she made in the past and intentionally leaving them being the last straw. Instead, they fully support Anne and let her go back to Amphibia without hesitation.
