I started a GBA Doom (1) editor some time ago. It is not yet finished but it is already usable to create a good mods.
- full texture/sprite replace support
- partial graphics replace support (images stored in WAD)
- sound replace support
- dehacked-like editing support, states and actors (with code-pointers)
- internal storage (ROM size) is limited, i recommend you to remove unused graphics, yes there is a plenty of unused graphics inside, before you start editing.
http://zdoom.ic.cz/zdoom/gbadoom.zip
Source code included - but it is not example of clean coding, reason included.
Hope this will help someone or ...
Spoiler:EDIT: to create a map you need a special app that will create GBA nodes called LEAFS, i am not quite sure where you can get it, but i know this app does exist. Someone already changed GBA map and created (found?) this tool.
