First of all, some stuff everybody knows:

• The content column of the #__modules table is only used by mod_custom and can only be used by com_custom.
• The admin component com_modules gives special treatment to mod_custom.
• A module developer has no way to get a field on the module edit page which will be stored in the content column because com_modules gives this field to mod_custom and only mod_custom.

I think this is a problem because some developer might like to make a different module type that uses the content field. Also, as a matter of code quality, I believe it would be nice if com_modules didn't need to know that mod_custom exists at all. It should have some other criteria (probably based on a value from the module's xml file) to tell it whether or not to show an editor and store a value in the content column.

Making this change is trivial, it requires only an addition to the mod_custom xml file and a minor change to com_modules in one or two files. But I'm concerned about changing the xml file. There doesn't seem to be a dtd for this file so there's maybe no issue with modifying it in some way but I'd like to know if anyone has any thoughts on it before I proceed.