Either way this was for a MMORPG where individualization was important. I dont think its as important in a solo game.
Depends on the player.
Visual mods tend to be the most popular for any single player games.
New character appearance options (Hairstyles, hair colours, make up, facial hair etc), new items and appearances and often options for customising appearances (Transmog, visual slots etc)
In addition, Gacha games are often single player and the major money maker is selling skins and new characters (Which are just visual options after a full set of SSR's is available to make a complete team)
Further to this, we also see plenty of games in recent times fail in part due to simply having bad character or equipment designs, where regardless of if they're multiplayer or single player, they get roasted to hell and back for not having good designs (While people praise titles like Stellar Blade for having great character designs, despite being a single player title)
Appearance customisation is a significant part of games for many.
BG3 for example has no such thing, except one can change the color of equipment. That doesnt do too much though.
It also has the camp clothes slots which function as quasi-appearance slots. Due to the option to have them override your actual equipment (Sans weapons). Which is the basis of many mods for the game, to add armour pieces and new items to put into the camp clothes slots to use as appearance slots for customised looks.
It's not ideal because it does affect your ability to have actual camp clothes, doesn't let you change your weapon and forces your weapon to disappear when sheathed (Which actually has mechanical impact in regards to using the Light cantrip)