Now that you have at least 2 ideas, it is now time to pick one. Remember, small but working is better than large and not.
Once you have picked the item you are going to make, the next step is to make a model of it. This IS NOT a 3D printed working example, it is a paper, cardboard, pipe cleaner, toilet roll, ... example. The purpose is to get an actually physically accurate shape so that you can see how it will work and if there are any problems that can be fixed, even before you start designing it.
Example:
Title: Raspberry Pi GameBoys
Group: I am a group of 1
Model: see below
What did I find out that worked: The larger size than the original Game Boy actually turns out to be good for me. Since it has a larger screen and I scaled everything up, I actually like the larger D pad and buttons.
What did I find out needs to be modified: The screen it turns out has a black bar on one side. This means I have to move the screen over to one side so that it still looks centered.