I've had this fork idea for a while now, its a lot of work but:
+Find a way to combine doom, doomII, final doom into a single wad then make a selector and put it on the GBA Doom ROM.
+Convert all music from both games, evilution and the unofficial plutonia midi pack to the impulsetracker format
+Coloured PSX lighting
+Improve the PLAYPAL GBA palette so not only is dark shading better, but sector lighting can be improved. Since the PLAYPAL graphic in doom has a few reused colors and some underutilised ones.
+Add the nightmare spectre in the port
+Port the PSX Fire Skybox in the game somehow, maybe if there's romspace just have it prerendered? +Fix the controls
+Make a save prompt at the end of a level?
+Manually optimise maps that cause trouble
+Find a way to combine doom, doomII, final doom into a single wad then make a selector and put it on the GBA Doom ROM.
+Convert all music from both games, evilution and the unofficial plutonia midi pack to the impulsetracker format
+Coloured PSX lighting
+Improve the PLAYPAL GBA palette so not only is dark shading better, but sector lighting can be improved. Since the PLAYPAL graphic in doom has a few reused colors and some underutilised ones.
+Add the nightmare spectre in the port
+Port the PSX Fire Skybox in the game somehow, maybe if there's romspace just have it prerendered? +Fix the controls
+Make a save prompt at the end of a level?
+Manually optimise maps that cause trouble