I know custom prefixes are only avaliable for partners and I respect that, but autogenerated names for prefixes trend to be so wrong, so I suggest to be able to select what part of your username you want as emote prefix, for example:

If your username is "JustAnnie" your emote prefix may be "Just444" or even worse.
Everytime you change your username or the first time you receive the affiliate Twitch should allow you to select in a text selector what part of your username you want to use as your name and let you to set an autogenerated prefix based on that selection, in this example "Annie444" or something more representative.