I would imagine a large complication to this entails a lot of the makeup from the Western side of things that were exclusive to collaborations, promotional events, and events that no longer exist. The instance I can think of is that the Dragon Hunter Frontier Z collaboration where I could envision the clothes and weapons could definitely be meseta pso2 provided a visual update together with everything else but that I don't exactly know how the licensing or how the legal side of the cooperation works where I'd wish it was as easy as Sega going through and updating the assets but I do not know about just how much of Capcom is involved in this process.
The same applies to collaborations such as the Final Fantasy XIV cooperation with Odin (which exists as far as I know) where I don't know just how much Square Enix is involved that for Sega to be able to upgrade assets there. When it was purely just Phantasy Star-related items (such as character outfits, cameos from different games, or other Sega-related properties) I could imagine this likely would have been done, or at least would be done. But in the worldwide variant we don't have the existence of Cougar NX so that I do not exactly know if there are other complications regarding Sega's own properties that might also contribute not to upgrading everything they could within reason.
I can imagine that probably out there we'll still see someone wearing those older collaboration items even when they are not rigged up to the very same standards as new makeup in a similar way and that there would have been outrage when it had been made obsolete or unusable. Collabs who have had their makeup removed still have their resources exist on the game so those will likely still be available for jp players.
Only improvements we'll probably see is the new engine improving shadows, shadows, colors, particle effects and projecting them against the outfits. You can have a look how exactly applying better shadows and colors can enhance old equipment by checking ReShade screenshots. Collabs who have experienced their cosmetics removed still have their resources exist on the sport so those will likely still be available for jp players. Only improvements we'll probably see is the new engine improving shadows, reflections, colors, particle effects and projecting them from the outfits. You can take a look how just applying better shadows and colors can improve old equipment by assessing ReShade screenshots.
The next step following Plains of buy Phantasy Star Online 2 Meseta Eidolon, to me, personally would have been Dragon's Dogma Online's open fields.