Custom fields are a very great tool, but also need to be managed carefully.
If everyone creates his own fields, it gets pretty messy very soon.
I suggest restrictions for standard users not to create custom fields.
I also would like to have an overall management and overview of all custom fields of all spaces. This table could be accessed by the account management and should include the possibillity to assign custom fileds to spaces, folders and users.
A nice feature would be an overview of how often a custom field is used, especially if it is in use or not.