My personal bias leads me to hoping that we can see some form of customization in something like the Photon Arts and a complete separation of things like the skill tree so that players can mix-and-match skills further than they already can.
In games like Dragon's Dogma Online, the way skills are handled are separated into three categories. Custom Skills are skills the players can utilize at their own will and are generally special attacks like how Photon Arts are in Phantasy Star Online 2. Core Skills are skills typically exclusive to the class and are always learned via leveling up and depending on the classes can be extended further. For classes like the Hunter, this would have been something like "Blocking" or the Gear Gauge mechanics. Lastly you have Augments which are purely passive skills learned via leveling up a class. All augments have a specific cost (and players have "Ability Points" to which they can allocate as space for these augments) and there is a maximum of ten augments you can have equipped. As a result, you can mix-and-match passive skills from various classes to specialize your character's build and general playstyle.
I would love if in the case of Phantasy Star Online 2 the skill tree simply was revamped so that you don't end up as a main-class Hunter who invested into Guard Stance and was stuck being a purely Guard Stance Hunter because you didn't have enough points to go into Fury Stance. Similarly, this would hopefully put an end to the Fighter being forced to play optimally by going in the singular direction of playing a low-health and extremely squishy build. With passive skills being equipable and freely put into conjunction with other skills there are definitely situations where specific class-based passive skills simply don't work such as in Dragon's Dogma Online where the Hunter's (bow and arrow class) passive skill that increases the number of arrows you get after reloading simply does not apply or work to any other class. Phantasy Star Nova already does something similar where leveling up a class unlocks skills that can be equipped to a "Skill Board" and having specific combinations of skills equipped upgrades the overall effectiveness of some skills. This system ends up being much tighter for higher-end gameplay as it pulls down the ceiling a lot and also at the same time widens the floor as there is a lot more in the way of making unoptimal decisions but for a skill-based game this does not bother me so much as the games like Dragon's Dogma Online tended to use this as a way to "normalize" damage against the higher level enemies to turn the tables from enemies being tolerable enemies who can fight back to making players run into a Souls-esque scenario where you're likely to die if you make a mistake during combat.
While I did not dislike the subclass mechanic in Phantasy Star Online 2, I really do feel that there was not much of an incentive to really utilize a class' weapons to its fullest of potentials when you can equip subclass weapons and still do relatively well. For some classes like the Techer this is a very big deal because the class is very heavily reliant on the subclass you choose in order to define its playstyle and capabilities. However, this leads into very big problems because classes like the Hunter in the global version provides "so much" utility and ease-of-access to basic quality of life features and in the past classes like the Gunner granted so much access to damage that was simply "too easy" to abuse. There is definitely a problem when you would pick a subclass only to utilize one aspect of its kit (in this case Chain Trigger) and it has resulted in the Gunner seeing numerous and repeated nerfs through the years simply because any class with relatively fast attacks and a very powerful burst DPS of some sort (infamously Braver/Gunner and Summoner/Gunner) were capable of nuking the "hardest" bosses at the time. To this end Sega forcibly put a ceiling on most damage which has destroyed the more hardcore aspects of the Gunner/Fighter gameplay and the Summoner's use of Marron (damage caps at 999,999) and ended up destroying the use of Chain Trigger outside of the main class (Chain Trigger Finish being a twin machineguns-only skill). Once again going to Phantasy Star Nova, the Hunter class gets a noticeable boost in attack power when utilizing just attacks and the Ranger class gets a noticeable boost in power by creating "chains" (which is simply done by attacking enemies as it contributes to a combo counting mechanic). I cannot recall what the Force specific gets out of its use of techniques but as you level up these classes you get the ability to equip a skill onto the Skill Board that lets you equip a certain weapon (such as "Equip: Swords" for example). Using these weapons off of the class they were intended lets you access the photon arts and techniques but you lose the main class mechanics that make them useful similarly to how right now our subclasses simply don't let you access main class mechanics. The "Buster" class in that game (the fourth class) ends up being a Jack of All Trade class with its own exclusive weapon (the Pile) and is capable of handling every weapon but without the bonuses and without needing the skills to equip those weapons. However, it ends up being a relatively "average" class simply because it lets the player shape it up to be whatever they want where the base classes are simply more specialized at their roles and naturally perform better at those roles.
Going to what I mentioned earlier with Photon Arts, I would love to see more multi-purpose use in photon arts or more customization like what the crafted techniques initially would have done. Dragon's Dogma Online decided to center itself around this in the end of its lifespan by introducing "EX Skills" that replace some Custom Skills. Upon learning them, players can equip either a "Power" or a "Technical" variant of a custom skill which grants a special glow and sound effect upon use and alters the use of the custom skill. For example, something like the Seeker's Explosive Flame Blade (on the ground it is a spin-slash where the player spins with daggers and releases power into the air before igniting it in a ring around the and in the air it is an acrobatic flip-slash that also releases power into the air before igniting the power) is only capable of being used once in the air and is one of their staple fire-damaging attacks. The "Power" version has a much stronger explosion that is capable of pushing its damage much higher than the original version and is a straight-up upgrade like how we can see the Technique crafting in Phantasy Star Online 2. The "Technical" version has the same power as the original but allows for a second use in the air so players can find another use for it to stay airborne or to use it for other goofy things (such as parkour tricks) which is similar to what one would expect from the Type-0 crafting for photon arts and techniques. I would love to see photon arts in Phantasy Star Online 2 take up a much more flexible role of being shaped for the player's playstyle and general use. This would definitely help to clean up the large number of techniques we have that all get dumped because Ilgrants is just simply too good and powerful for what it is or to help bring variety to classes like the Gunner and Fighter who have a large number of photon arts but not enough buttons to use them all (or in the case of the Gunner the lack of a reason to omit Infinite Fire Type-0/Infinite Storm Zero from any damage rotation involving breaking their Chain Trigger). I would love to see an expansion on the way the Successor classes utilize the Photon Arts because each of these classes do it in such a way where all of their photon arts fit and you have access to the entire kit at all times. However, for the customization capabilities it most definitely does not exist due to the fact that all of these classes only vary among players based on playstyle and the fixation Phantasy Star Online 2 has with optimal damage tends to neglect playstyle quirks in favor of relatively easy and repeatable mechanics (such as the Hero's reliance on cycling attacks until you can use the third normal attack for the Sword and Talis). The Luster has "one" more photon art that they will run out of space for on a controller or on your weapon palette but this is a purely optional photon art ("Smart PA") which essentially is an auto-attack button for players who are overwhelmed with the Luster's combination of the Successor class mechanics with the older class gimmicks as well (Enhance Arts, Move Arts, Stay Arts, and so on).
My hope for New Genesis is that the classes (old and new) have a relatively even level of ground between them all so that we don't end up with the situation that the Japanese version has always had where classes like the Force ended up outdated extremely early on and were unusable as a Subclass (because they offered nothing of value) or that we have to wait for new classes (like the Phantom and Etoile) to come around and act as a band-aid to make everything play more smoothly without touching the new gameplay and mechanics. As much as it is a joke for a lot of people I feel that there is something very off about players who play as a Summoner cheesing content by doing absolutely nothing and you have classes like the Fighter on the other hand who are putting just about everything on the line to simply get wiped for a mistake they made.
And lastly if the Summoner was to return I would love to see them utilize their pets in a similar fashion to Astral Chain's normal combat.