There are around a dozen issues related to this on the tracker. The current implementation is not very flexible, and relies on the user knowing how to configure xdg-stuff to get the result they want (and it’s not documented in the wiki). Additionally, it’s mostly broken on Windows. Not good.
If this kind of customization is added, the question is how to organize it, and the associated preferences.
Features that could be added:
User can type in a command or path to external app (manually, or using a file browser). These entries will be stored in the user preferences and can be removed/edited (either under a new tab in preferences, or directly in the list). Should probably be listed separately from other entries, or otherwise marked as being user-provided.
Configurable exclusion lists
User clicks on an x-button, and the associated external app is ignored (placed on an ignore list, where it can be subsequently unignored). Not 100% necessary in my opinion, but a nice-to-have. Trade-off is interface complexity and maintenance.
User uses only, say, GIMP or Krita for 90% of their external edits, so they should be able to avoid having to initiate the edits via the selection window, while still having the option to do so if necessary. Another nice-to-have, but with trade-offs similar to exclusion lists.
We also have the issue of not supporting automatic updates when editing externally on Windows, but I would consider that to be a separate problem.