It would be nice if the user could just make an avatar model, give it an animation skeleton, give it animations with specific names tying fill in a txt with various attributes of the character, optionally, so only the attributes you put down would be changed. For example, if by default the player has 100 health, weighs 20 pounds, and can walk at 1.5 m/s, in this text file you could put
playerHealth = 300;
playerSpeed = 2;
and then your new character would have 300 health, weigh 20 pounds, and walk at 2 m/s.
The same could go for animations. Any animations that the player file doesn't have in stock with specific names would just resort to the default animations.