With many creators offering uploading avatars to people's accounts as a method of selling them, I believe an option to make an avatar accessible to specific users via their user id or display name would be extremely helpful.
I believe this feature is extremely important, because it allows people to create avatars that only specific people can use, without having to go through the hassle and unsafe method of sharing account credentials and uploading the avatar to their account. Specifically for avatar sellers, this would allow them to release avatars only to their supporters for use in a much easier way. For example, if someone wants to create patreon-only avatars, they would be able to do so by whitelisting them on the avatars that they so please. It would help support creators' monetization more, as well as just offer a simpler method for friends to share their creations with people that they know. It makes sense for people to want to keep an avatar private to select groups, but currently there is none such method other than sharing the asset files or account credentials to upload to others accounts.
For the sake of account safety and convenience, as well as for avatar sellers, the ability to whitelist users to grant (or remove) access to wearing an avatar would be amazing for the game. The same method could even be applied for worlds as well. In terms of implementation, I believe a new setting in the sdk called 'friends only' or 'whitelisted only', with a text box allowing users to enter userid / displayname would make this easy. It would also allow creators to not only add people to the list, but remove them (ex: in the case of sellers, a previous patreon supporter was not subscribed anymore; or the creator simply didn't want someone to use it anymore.) Fingers crossed! Make this happen (: