In my eyes an SDK is made for people knowing what they do. Modding requires some skills. The game code can be easy decompiled using "unluac" (its a java app), it allows you to edit campaign script, monster AI, interfaces, spells, traps, whatever. Which is a great start. Releasing the original file would also provide formatting and developer comments, but optional in my eyes.
Summary of what i have:
GameScript:
Use this to edit the game script.
- unluac "Decompile the GameScript" ->
http://sourceforge.net/projects/unluac/
- SciTE (edit and recompile GameScript) ->
http://www.scintilla.org/SciTEDownload.html
Also it would be nice for an official github repo with the original scripts, so comments and such are present. Also forking for mods and forking mods themself would be easy.
Editing or adding textures and other images.
- photoshop .dds tools ->
https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop
Edit or create maps.
The .gnd files are still unkown. It could be easy (but take some time) to reverse engeneer it and write an editor, but the devs will have one, so it would be awesome to see it released.
Models
.cef and .hkx importer / exporter for 3dsmax, cinema4d, blender, and some others? Can also be done by the comunity, if the right people get started with it.
Fonts
As fonts are char-maps in .dds format, they look like having an .fnt file, which maps the characters to the coords (i think). Having a tool to create it or generate from .ttf or such would be nice. I think the devs also have this, otherwise its a small command line app someone could make.
UI / GUI
All files are in xml syntax and easy editable. Some documentation or editor would be nice, but working with the xml via Sublime or such is basicaly fine.
Sounds
Just convert them to .ogg and they work.
Localization
I made a decrypter and encrypter for the .loc files, which allows to translate the game in every other language, or simply add strings for your mods. Will upload this tool along with the others as soon as they are ready.
Unknown
Whatever the .db files are and include, some information and / or editor would be nice
And at last:
What needs to be added to the game?
Mod support. Nothing much, just add a mod folder as primary source to the resource manager internaly, if usinf physFS, it can also simply read out of .zip files, so every mod can be a zip, including a small .xml file with mod name/version and such. And a md5 checksum can be used to compare if all clients have same mod.
I hope to see a solid modding SDK comming, As this allows to implement a bunch of new things and keep the game alive longer and longer without the need of the original devs to add content to keep people playing it.