From Assassin's Creed?
Also, perhaps there could be a database of skins of popular YouTubers, players, and overall generic skins such as animals, people in suits, headphones, etc. and then also a customizing system, where it takes a separate face/head, headgear/hat, clothing, etc. and matches it randomly.
Cannot be tabbed - That'd be good, but when you're trying to check their stats it could be confusing.
The other two I really agree with as well. It's annoying when people see you left, and the usernames are super easy to tell who's disguised, very often. Sometimes people get lucky and end up with decent ones, but they always have numbers which is a bit... err.. well, I think it would be good if they sometimes didn't have numbers, or maybe more/less.