The biggest problem with most hacks or games is that they are doing nothing but adding Fakemons or changing the skins and that's all. However, you need at least one good idea which is totally different from the usual games. In case of a Pokémon ROM hack, or a totally new project, you can go with a totally new genre, or stay in the basics and modifying the details. First of all, you need to decide what kind of game do you want. The planning is the most important part of creating a (fan-)game.
