Something I have mentioned elsewhere numerous times already is that I would love to see some return of the Skill Board or at least a very big separation of what "skills" do for the classes. As in I would want to have the "core" skills for classes be something you shouldn't have to sacrifice (All Guard for the Hunter, the Focus/Gear skills for weapons, and things like the Luster's "moveset" that unlocks as you level up) and so that things that are more on the passive side of things (like Stances) can become optional to equip and mix-and-match with other similar skills.
The big reference point I would have for what I would like to see stems from Dragon's Dogma Online where the "Custom Skills" are equipped to a palette just like our Photon Arts are and largely determine how you enact your playstyle, the "Core Skills" are all baseline functions that your class should be able to do that give them an edge over other classes and their essential core gameplay (things like the Gear Gauge, Chain Trigger, the Phantom Marker, Hero Time, elemental swapping for the Techer/Bouncer, and more), and lastly that all passive skills are counted as "Augments." Augments are unlocked and typically are specialized for shifting the behavior of a class such as loosening and relaxing the timing of a just-guard to things that can potentially change gameplay such as adding striaght-up unconditional damage boosts.
However, in Dragon's Dogma Online, players are limited to what I recall was something along the lines of eight or ten of these being equipped at a time and in addition they each cost a specific amount of "Ability Points" (AP) to install which meant that you pick all the damage-based skills but have absolutely nothing in terms of quality-of-life gameplay conveniences. This meant that the unconditional damage was the lowest effort to play but came at the highest cost (with little room for leeway in customizing your build) and that going a high-health build meant you had to maintain your health in order to maintain that damage. However, a high-health build was undoubtedly stronger than the unconditional boost and granted some room for more quality-of-life changes but it was still very costly. The low-health build in that game was easily the cheapest but the most dangerous option due to how it was difficult to maintain (allies can heal you past the point where the build would have worked) and gave you the potential to stock all sorts of quality-of-life changes and tweaks to really shape your playstyle further. In that game, you can play as the Warrior (Greatswords) who had a mechanic that allowed them to reduce the damage taken upon both attacking and being attacked at the same time. The act of taking damage during your attack animations filled up a meter that can be expended to deal heavy damage (typically damage-cap levels of damage) but with the right combination of augments and the low-health build you can reduce the damage you take by up to 90% which typically turns "1" damage into "0" so you can effectively be immortal so long as you play properly. Keep in mind, these are "options" for how a player can build their class so that there could be another Warrior who wishes to simply play more traditionally and safely while still dealing that high damage though in a slightly different means.
My hope is that Phantasy Star Online 2 (and hopefully New Genesis considers this) takes a few pages of how the Skill Board from Phantasy Star Nova was very similar to what I described from Dragon's Dogma Online as leveling up classes unlocked skills that can be used on the Skill Board and allocated. It had limited space and specific skills synergized well including skills from other classes which created "Skill Combos" that boosted the effects of equipped skills. The classes you play as already would have some of these skills be innate (such as "Equip: Swords" being naturally innate for a Hunter) and using weapons off of the main class resulted in the player not having mechanical features those weapons would have had (Hunter's Just Attack multiplier bonuses, the Ranger's "Chain Combo" bonus, and so on). A large part of that is that there was also a universal class (Buster) that utilized every weapon but without the bonuses except for its own exclusive weapon (the Pile).
I would love to see and anticipate that subclasses in New Genesis will be much more polished than they currently are and would absolutely not mind if the subclass offered nothing to the main class at all outside of access to more weapons and gameplay opportunities rather than pure stat boosts and utility.