To play devil's advocate here, I think that when fans ask for things like "a shop" what they are really saying is that want to lose themselves in a game they love.
Remember how a nostalgic game from a now forgotten time could really capture the imagination? Before the internet, tv and video games were as close as you could get to a digital eco system.
So, a game's attract mode, or even that game manual you'd got in the box with a new home game. All that fun stuff added to the experience when done right.
That said, have you considered a virtual in-game manual to your game? Maybe in the style of a parody of a Genesis game manual?
That could be fun, and even useful, if you use it to educate a player on what moves to use and stuff like that. Depending how far you take it, you could even do a little blurb about each enemy and so on and so forth.
As an example, I did a virtual fully functional game manual for the "How to play" section in my menu for my WIP Double Dragon game I'm working on, and even added silly gags for that nostaglia fix (i.e. Red Bull is now Red Ox, Cheetos are now Cheezoz and etc. and etc).
Those little flourishes, for me at least, deepen the joy and expand the world a game seeks to create. Just sharing that, and my 2 cents, for the fun of it.
