Well Roblox's catalog, or shop if you will, uses something called Characters which are similar to s&box's Model selection in the avatar editor. And Roblox players can create base model characters, similar to s&box's Human and Citizen. With these Character base models, users can decorate them with clothing and accessories like you would with the Human and Citizen models
If one wanted to be Miku in s&box as an example, instead of having a standalone model with the clothing not being separate from the body. Users can create a female anime Model, Miku's clothing, and hair so they can put them together creating a Miku avatar.
Also just to let you know, I don't know if Feature Request is for the sandbox gamemode, which i'm assuming is a game inside of s&box. My UGC model concept was for the public shop where players can post accessories and customize their s&box avatars. And I have never laid a hand on s&box since I am on a MacOs and plan to get a Windows PC soon, I'm just using my knowledge from the community videos and Reddit to create this post.
Really I don't know how clothing shop works on s&box, I just thought adding custom base models would be pretty sick since it can give more freedom for the players. But I totally understand if my UGC Model concept doesn't have potential or won't be doable.