From Sora's perspective, the villains that have paved the way to Kingdom Hearts 4 have all been bumps in a long road of Xehanorts obsessed with the power of Darkness. However, one of these variants has had a much more personal antagonistic relationship with a different member of the Destiny Islands trio, acting as a powerful foil for Riku's path from darkness to light.
Given the impact of Ansem, Seeker of Darkness (the Heartless one) on Riku's character growth, the members of the main trio should have their own personal villains in Kingdom Hearts 4. This could be difficult for Sora, given that his motivations are always about saving the realm of light, high stakes that don't generally allow for a very personal connection to the villain. Instead, Riku and Kairi still have quite a bit of baggage to work through that could be heightened by a villain that acts as a foil for these specific struggles.
Riku's History with Ansem, Seeker of Darkness
The personal connection between Riku and the first antagonist of Kingdom Hearts, the heartless form of the villain who would later be named Xehanort, runs far deeper than any other hero/villain dynamic in the series. Starting from the original title, Ansem's dynamic is immediately deepened by that fact that he acts as a trickster from the very beginning, leading Riku to isolate from his friends and succumb to the Darkness. Riku's mistake haunts him for the rest of the series, and is personified by the continued conflict with the villain lingering within the hero's heart.
This was a thread that persisted from Chain of Memories and through to the Riku segments of Dream Drop Distance. By the end of this ongoing plotline in Kingdom Hearts 3, both Riku and Ansem share a vulnerable moment of seeming respect to see the other go. It was a great cap on a strong villain, but it does leave Riku without that personal connection to the antagonists as Kingdom Hearts 4 prepares for a whole new story arc.
Riku's New Struggles Personified
Now that Riku has overcome the influence of Ansem and the Darkness that came with this original villain, the character still has another struggle to overcome. Although Riku has already tackled his insecurities when it comes to being second best to Sora, he also still has fewer solo victories to himself than someone might expect of one of the strongest Keyblade Masters in the series. As such, it isn't exactly his competitions with Sora that Riku needs to overcome, but rather the doubt that comes from a series of losses throughout his adventures.
Kingdom Hearts 3 Re:Mind might have already introduced the perfect antagonist to pressure this growth in the form of the secret boss Yozora. While not seemingly a villain, Yozora could take the same role as foil to Riku that the original Ansem did throughout the first three games. As a character who never loses, Yozora could keep Riku's losing streak consistent, right up until the narrative swell where Riku has to confront his own fears of loss comes and the Keyblade Master earns a decisive victory.
Regardless of how the villain story plays out, the personal connections that the Kingdom Hearts series previously had between Riku and Ansem isn't one that should be forgotten. Supporting characters like Riku have more room for depth to explore these narratives. Personifying these struggles through a strong villain or antagonist is a large part of what makes these games so beloved by fans. Then there is Kairi, who's similar struggle to find a hard win on her own could help give Kingdom Hearts 4 a hint of balance, as both characters are given antagonists to embody the personal conflicts they need to overcome.
Kingdom Hearts 4 is in development.