bug 1432 addressed a number of different requests. The bug there in general is useless, however there was one potentially useful feature idea.
Perhaps a method of linking user accounts back to a primary account. The use of this feature is limited only by local policies. On wiki that allow use of multiple user accounts, this could be use to connect a group of accounts together to one user. Or on wiki like Wikipedia this could be used to allow a user to link bot accounts to their user account.
But extensions could even hook in and add extra functionality, perhaps for things like auto redirection of talkpages, or making the new-message notification show up for all users.
The one builtin feature for this involves user subpages. An account would be able to edit the .css and .js subpages of any account linked to the same master account as it. This way a user would be able to edit the interface stuff for it's other accounts. Such as their bots, without needing to change their login. This could be especially useful if extensions like [[mw:Extension:KeepYourHandsToYourself]],
[[mw:Extension:EditOnlyYourOwnPage]] were installed, because those extensions limit user page editing even further.