Currently, adding new modes involves some pretty complicated steps:
1. Putting the files in the right place
2. Editing the catalog
To make this better, jEdit should offer a "Install mode" menu where users can select a file which jEdit them puts int he right place.
Also, jEdit shouldn't depend on a catalog file or at least it shouldn't require users to edit it themselves.
I suggest that jEdit should load all modes automatically. Please move the FILE_NAME_GLOB option to the root MODE element of the mode.
If a mode consists of several files, they should be put into a single JAR file with the XML being in mode.xml in the root of the archive.
Dropping a mode file/archive into the mode folder should be enough to install it. If necessary, you can add a prefs page where modes can be en-/disabled.
It would also be nice to wrap the loading of a mode in a single try-catch and disable the mode if there were any errors loading it. Currently, problems in the mode. I forgot to copy a DTD file that was referenced 50 times and that forced me to click 50 error dialogs...