What do I do first when I create my ROM Hack, eh? Well I would pretty much create the new tilesets, then work on the sprites, then start on the custom music, then start some ASM work (such as removing the motion of the truck in the intro of Pokémon Emerald, but I have to learn how to do it first), then start the mapping and scripting and make sure everything's in correct order.
For what I'm planing in the future... is Pokémon: Dimensions of Time.
Which will be a hack of Pokémon Emerald. Ohh yes! Just to bring out that leaked information of my hack, I've only just started with the tilesets, so it's still far off in development. ;D