If Frozen 3 ever sees the light of day, it will need to give Kristoff more time and impact in the story to make up for his sidelining in Frozen 2. While there's no guarantee that Disney will make one of its biggest hits into a trilogy, plenty of theories and expectations abound as to what the third installment of Frozen should look like and include if it ever happens. One of the foremost hopes is to see Kristoff recover some of the ground he lost in Frozen 2.
Frozen is, of course, primarily about Anna and Elsa's bond and character development as sisters, and about Elsa's journey of self-discovery, of which Anna is an important component. In many ways, the series eschews romance, since Elsa remains single and Anna's initial picture-perfect Disney romance with Hans turns out to be a horrible trap. Thus, Frozen's real love story is Elsa and Anna's sister relationship, and it tends to take precedence over everything else in both the original film and in Frozen 2.
Yet, Kristoff shines in the first Frozen as a refreshing departure from the usual suave, heroic, charge-taking Disney man, and his eventual romance with Anna is marked by a healthier, more realistic dynamic in which they don't fall in love at first sight and they feel like true equals. Aside from this, he also has his own interesting backstory. But Frozen 2 seriously sidelined Kristoff, which means Frozen 3 will have some work to do giving him the time in the spotlight that his character deserves.
Although one of Frozen 2's best moments is Kristoff's song "Lost in the Woods," this is virtually the only moment of development he gets. Likewise, everything he does in Frozen 2 is in service to furthering the Anna-Kristoff romance plot. But just as Disney princesses deserve to have a life outside their relationships, so too do their male counterparts. The original Frozen seems to set Kristoff up as a very important and interesting character from the time he is first introduced, given that his family is somewhat of a mystery and he was raised by trolls. Should Frozen 3 ever come to fruition, there's plenty of potential for the story to do justice to Kristoff by explaining his backstory and troll family further, or at least giving him more screentime and significance besides chasing after Anna. Since Frozen and Frozen 2 have spent a lot of time developing Elsa and Anna's story, Frozen 3 could be the perfect opportunity to focus a bit more on Kristoff.
Regardless of what direction Frozen 3's story takes, and whether it actually happens, it's clear from Frozen 2's treatment of Kristoff that the character has more to offer and deserves to be featured more prominently. He's one of few Disney princes who let their princess call the shots, and has an interesting past ripe for unpacking. After his unfair sidelining in Frozen 2, Frozen 3 has to do justice to Kristoff in order to keep the story interesting.