This can only be accepted when creating a new character. Here are some concepts that could be involved:
-EXP/Rare Drop/Meseta boosts of any kind are deactivated (PSE bursts, items, Drinks etc.)
-EXP gains from client Quests are removed, in turn provide Meseta
-Photon Art skill levels increase by using them and leveling them up (like in PSU) instead of finding higher level discs
-Upgrading weapons no longer require meseta and additional weapons, instead just grinders, but grinding can still fail and possibly break.
-Damage scaling reworked in this mode to be alot lower (doing 4 digit damage is considered endgame level)
These are just some concepts i thought of that could make this game mode feel more similar to the previous PSO/PSU experiences.
Basically leveling up would be mainly from rerunning missions and killing monsters over and over again aswell as hunting rare items being actually rare since rare drop rate bonuses are removed. Leveling up photon arts is reminiscent of PSU allowing you to focus on the abilities you prefered the most and power them up.
Any other suggestions would be appreciated The goal is basically to make the grind slower and more rewarding drop wise