Start somewhere, anywhere. Even if you don't know what you want to make, make something until you do. You said you made an emulator to run roms rather smoothly, so why not make an engine/framework for other people to use for making games? You'd get some practice in while you figure out what kind of game you want to make and everyone else would have a new tool available to them. Maybe try some kind of Metroidvania or overhead Zelda type setup?
Starting very low would be probably the best: make something like pong. In my opinion it is a good practice if you are using a new framework/engine. After that, go for more difficult projects. While practicing you may find something you like, or so, what you want to put in the game that you want to publish.
(I did it the other way around: I started with a "hard" project [hard for beginners] and then did a game like pong for a school project. It worked wonderfully.)