I have a couple suggestions on how you might be able to get custom skater models to work in a similar way to how custom levels work now (re. without having to replace anything). The first is the least elegant but still perfectly serviceable and likely the easiest to implement (I say likely since I don't know the first thing about making/modding games), and the second method is more graceful but, I would imagine, considerably more tasking to accomplish.
The first idea is that, after the main skaters on Character Select, you could have a set of 20 or so cloned placeholder skaters (maybe Tonys or something) so that you can have a set of custom models without having to worry about replacing anything - online you would only appear to be skating as Tony, unless the person viewing you has a different model assigned to that particular Tony. It might also be possible to set names for them once you've assigned a new model to them?
Alternatively, you could make it work similarly to the new levels, with the .json files and everything so that they're just added to Character Select as you download them. As for the issue of how they'd work online, I thought perhaps you could have it so that the game detects that you're using a custom skater and replace it with a default character for observers (could be difficult to implement) and have a prompt before you play that says something like "You are using a custom player model. Other players who do not have this model downloaded will see you as [x default game character]" with maybe an option to change what that character is?
