While Kingdom Hearts 3 missed out on several opportunities, one that stands out has to be the Coliseum Cups introduced in Kingdom Hearts 1 and 2. Square Enix decided not to add these activities to Kingdom Hearts 3 despite the game reintroducing their hub world of Olympus back into the series. Considering how popular and creative the Cups were, this decision was an unfortunate turn of events, especially since it's not clear why it happened in the first place. Still, it's never too late for Coliseum Cups to return, and they should in Kingdom Hearts 4.
If the Kingdom Hearts series' near 20-year history is any indication, these activities hold some of the most epic battles players can participate in. Also, in returning Olympus to the world lineup, Kingdom Hearts 3 introduced several new elements to it that Kingdom Hearts 4 can take advantage of when revamping the activity, similar to what Kingdom Hearts 2 did when it inherited the Cups from Kingdom Hearts 1.
Zeus as a Boss Battle
The venue and host of the Coliseum Cups change with every mainline Kingdom Hearts installment. In Kingdom Hearts 1, Phil held the contests in Olympus Coliseum, while in Kingdom Hearts 2, Hades held his own version of the competition alongside Pain and Panic in the Underworld. Now that Zeus has appeared in the series via Kingdom Hearts 3, he can hold his version of the Coliseum Cups on Mount Olympus, which also made its first physical appearance with Kingdom Hearts 3.
Just like what Phil and Hades did for their Cups, Zeus can title his events after himself and his allies, as well as add his own rules and stipulations to each tournament - ones that play off of whatever new mechanics Kingdom Hearts 4 introduces. Now that Kingdom Hearts has explored all of Hercules' major villains, they can appear throughout the tournaments too.
More importantly, Zeus himself can join the fray. There's probably no one else that would make for a more exciting opponent for players to face off against in the Coliseum rounds, and it would be fitting to see Sora, Donald, and Goofy tangoing with the head honcho of Olympus now that they've proven themselves to be true heroes. Kick it up a notch and have Sora face off against both Zeus and Hercules at the same time, and that would be a match fitting to close out Olympus as a world for good.
Final Fantasy Fights
Outside of Sephiroth in Kingdom Hearts 2, the Coliseum Cups serve as the only places players can meet different Final Fantasy characters in combat. In fact, Olympus stands as the only Disney-based world where Final Fantasy characters ever appear. Beyond it, Kingdom Hearts confines them to its original planets, such as Traverse Town and Radiant Garden.
They likely only appear in Olympus at all because of their participation in Coliseum Cups as Olympus serves as the perfect world to hold exhibition and tournament matches. Said matches play to the theme of the place, and Kingdom Hearts doesn't have to waste time fleshing out new arena-based worlds for these matches to take place.
With that said, Kingdom Hearts 4 bringing Coliseum Cups back creates opportunities to introduce new Final Fantasy characters for players to square up against, and it's long overdue for Kingdom Hearts to bring in new FF characters. Zack Fair was the last new FF member to join the series, and not only was that more than ten years ago but the post-credits of Birth by Sleep — the game he appears in — suggests he won't return in any future installments, so he hardly even counts.
All in all, it would be interesting to not only see new Final Fantasy characters appear in Kingdom Hearts 4 but for them to appear in Coliseum Cups for players to fight against. Perhaps Lightning from Final Fantasy 13 can enter the ring, and depending on whether Yazora is his replacement, maybe even Noctis can appear, too, as well as Clive Rosfield from Final fantasy 16.
Mirage Arena
Olympus can't stick around forever in Kingdom Hearts. It's already exhausted the adaptable material of the Hercules movie by this point. Hence why Kingdom Hearts 3 would have been the perfect opportunity to send the world off with Coliseum Cups that pull out all of the stops. But even when Kingdom Hearts eventually does leave Olympus behind, it doesn't also have to ditch the popular tournament-style activities the world introduced because Mirage Arena still exists.
Mirage Arena served as an alternative to Coliseum Cups in Kingdom Hearts Birth by Sleep. Terra, Aqua, and Ven used it to test their mettle against the numerous Unversed they encountered in outside worlds, as well as some new Unversed local to the Arena. The world even adapted the Gold and Platinum-style matches from KH1, throwing players against secret bosses like Monstro, Armor of the Master, and No Heart - all of which added their own charm to the game.
That said, it's worth noting that Mirage Arena is not nearly as lively as the Coliseum. Unlike Olympus, it lacks a backstory and so primarily exists to facilitate Birth By Sleep's multiplayer. That said, it does what it's supposed to do well enough, so if Kingdom Hearts Director Tetsuya Nomura decided that he wanted to use it as Olympus' replacement when it comes to tournament-style events, he could.
The Kingdom Hearts director could then just develop a backstory for Mirage Arena afterward. It could even be something as simple as describing the place as a training ground for Keyblade wielders. The original plan for Arena was to give it a backstory anyway, one that saw Final Fantasy 8's Laguna Loire appearing as the host of the venue.
Regardless of where Coliseum-style content appears, the overall point is that it's a waste for mainline Kingdom Hearts titles to not take advantage of them. Not only do they add interesting side content that tests the player's skills throughout the game, but they also offer interesting ways for Final Fantasy and other Square Enix-original characters to get involved with Kingdom Hearts outside of whatever world acts as the hub for the current game.
Kingdom Hearts 4 isn't confirmed to be in development.