Figure out how to handle preferences that, on a wiki farm, only make sense as global preferences.
An example would be LoginNotify email alerts of a failed login attempt when CentralAuth is enabled. (cf. T174263)

This could be done with GlobalPreferences using a hook to override the local preference and always load/save the entry in the central DB preference table. If they appear on Special:Preferences, the description could get appended a "Global" tag

samwilson mentions that in addition to 'noglobal' (T174099) we could also do 'nolocal' and 'nolocaloverride':

• no local means no local pref at all (if GP is enabled)
• no local override means there is a local pref, and a global pref, and you're not allowed to override the global with the local