No details are known about Persona 6 right now, so everything about the game's potential is primarily speculation about what direction fans would love to see the hit JRPG franchise go. Persona 5 was the highest-selling game that Atlus has ever released, which puts enormous pressure on its follow-up to deliver more of what the players love while still keeping things interesting.
Each Persona game has made various changes to refine the formula or shake things up for fans who want to experience something new. Things like the weather forecast system introduced in Persona 4 or the lightning-fast combat and eye-catching art style of Persona 5 helped make those games unique and innovative. Most games in the series have one thing that has remained relatively unchanged, which is where Persona 6 has an opportunity to make a big addition.
Persona 6 Needs More Personas
The Personas within the Persona series, as well as the alternatively titled demons in the sibling Shin Megami Tensei series, have remained somewhat unchanged over the years. Each game has certain additions, but players can always generally expect to find the same creatures as they begin their adventures. Pixies, Jack-o'-Lanterns, Agathions, and many others have been standards since the early days of the popular Atlus JRPGs.
There have been additions throughout the years, typically to the higher-level Personas that can be fused using the lower-level Personas. Many of these stronger creatures still tend to be repeated, though, like Melchizedek, Alice, and Beelzebub among many others being found in nearly every Persona and Shin Megami Tensei title. Persona 5 Royal has 259 Personas in total, 33 more than were available in Persona 4 Golden. That does add a certain amount of variety to the game, but for the diehards who have played every Persona and SMT game in the past, the roster still likely felt largely familiar.
Persona 6 Could Take The 'Unova' Approach
Game Freak diversified its own portfolio when it added the biggest new crop of creatures since the first generation in Pokemon Black and White. The first games of Generation 5 had no Pokemon from prior generations available to be caught in Unova. This was somewhat controversial with players at the time, especially ones who were critical of the new Pokemon designs, but in retrospect, it was a great move to make the new games feel fresh. In the current "Gen 1 Pokemon saturation" era, it would be nice to boot up a new game and not immediately be bombarded by the same 150 from Kanto everyone's seen many times before.
Persona 6 could feel extremely unique in comparison to its predecessors by pulling a similar move and adding a 100% original crop of Personas for players to negotiate with and fuse together. One of the few downsides to this is that it would likely take a long time to create the hundreds of new Personas necessary to pull that off. Persona 5 was already hit with a lot of delays without trying to fundamentally change its inventory of Personas. Persona 6 could spend even more time in the oven if Atlus tried to pull something like this off.
Each Persona is also based on some kind of creature from real-life folklore or mythology. Considering there are hundreds, it could be a challenge to find enough other mythical creatures necessary to create an entirely new roster from scratch. It's not impossible, though, and there would be a lot of interesting cultures to pull from that were never referenced before if Atlus decided to do this. The current lineup of Personas is largely inspired by European and East Asian folklore, and a new lineup featuring more creatures inspired by the myths of South America, Africa, Oceania, and so on could be a welcome addition.
Persona 6 is in development.