I've made mods both out of what I want to see in the game, as well as from not wanting to have to wait on the bigger mods to update for me to have certain functionality. Don't worry about reinventing the wheel, just make what sounds fun to you, and worry about whether you want to release it later. I never released my early stuff, and a few smaller things in between. It'll take some time to learn your way through the game's guts, so you might as well be making something fun in the process, even if that's pointlessly making exploding pigs or creepers that plant flowers. "Fun" doesn't always translate to "useful."