Just gotta keep the scope smaller is all. Metal slug had about a bazillion frames of animation and high detail to the graphics. The amount of time spent building so many frames let alone animating them all will run the time on the project right up. To do that you would definitely need more than one artist working full time on it.
So by comparison a game like gradius or any other horizontal space shooter would be very light in scope. There is FAR less animation and far less frames for each object as a result. Because of that doing a game in that style is much much easier to handle as a solo developer. This is just one example though. Another great example is castle in the darkness. There is very light animation involved in it and the game was developed by one guy. It has the exact same thing in common with the first example.
FPS is not actually a bad idea it might be a bit heavier in scope than my examples here but it also isn’t a huge thing like metal slug would have been. You could realistically build a FPS that is playable and continue to expand on it over time with either patches or future games that can take advantage of the previous work you have done. So while the first game might not be exactly what you want it can easily get the ball rolling for bigger better projects over time thus eliminating the issue of having to pour all the time directly into a single release all at once.
Just remember though you aren’t alone with this issue of scope and trying to find a good starting point is something many many of us have done. Just keep asking the questions you are because you are on the right track and there are a lot of helpful people around here to guide you along.