Graphics editing - I'd like to get a basic background tile editor working. Something similar to the mmx editor where there's a tile editor, map editor, block (?) editor, and scene editor. Sprite editing is more difficult and I don't have time to spend on it right now. I'd like to display enemy/object sprites on the map and allow for moving, adding, and removing them, but probably no editing.
Palette editor - This is straightforward to do for at least the background tiles. Most of the palettes are not compressed.
Uncompressed size - I haven't looked at how well the compression works. My guess is that the average compression ratio is not better than 2x (uncompressed to compressed ratio) so it would be < 3MB (1MB original ROM + 2MB uncompressed data) total in size as a worst case. I wouldn't be surprised if the actual ratio is closer to 1.3x and not all of the ROM is compressed so it may be less than 2 MB total. Someone could probably dump uncompressed data in the 1+ MB area of the ROM and branch before the decompression code to a MVN.
The editor probably won't be capable of any actual editing for a while. I need to figure out the various graphics mode code (mode 7 levels crash right now) and get more of the levels displayed correctly. Collisions, events, sprite display, more details on the level segments are constructed .... there's a lot of stuff that is still a mystery.
EDIT: Has anyone worked with mode 7 before? I read up on it and the non-extended version of it has just one 256 color background. But when I go into snes9x and toggle on/off the backgrounds it still behaves like there are 3 (foreground tiles, background tiles, status bar) like it does in mode 1. An example of this is the very first level segment in SC4 where you jump on the drawbridge which rotates up. Maybe that's just a trick based on how they prioritize tiles and snes9x is aware of which tiles it needs to enable/disable?