Something like this has been requested at times. I'd like it too. It would make MC into a total media database manager.
One of my uses would be to catalog media that for various reasons isn't digital and/or isn't directly available to MC. I'd like to catalog the home movies in the box in the garage, and the vinyl albums on a shelf, etc.
Another use would be to catalog wish-list and to-do items -- things I want to add to MC, but first need to track down, or digitize, or buy, or whatever.
In both cases, MC would need to allow a database record to originate by adding a blank record and typing in info (or copying a record and editing info), without starting by importing a media file.
Perhaps this could be implemented via a data record "Status" button that tells MC about the record's media file. Maybe it has values such as Local, External, and None. If/when a "None" file becomes available, it could be added to the existing record by changing the Status to "Local" (instead of having to create a new record) and adding the file's path to the record (same as cover art is added). (A value of External or Remote or Removable could indicate the media file is only available to MC some times.)
Allowing an MC database record to exist without an attached media file could also help the situation of changing the file associated with a record, usually to replace a poor rip with a better rip. Just change Status to "None" or whatever, replace the file, and change Status back to "Local". Probably this would be accompanied by a way to select and attach the new media file to the record, similar to how cover art can be attached.
I hope...