Hola Stuart,

It would have a different nuance if you use "sería" instead of "podría ser".

These are the two contexts when using it with podría.

1. Yo podría ser Superman. (I could be Superman [because I have the right skills to be Superman, e.g. I can fly and I am very brave]

2. Yo podría ser Superman. (I could be Superman [...and you could be Catwoman] This is just giving a suggestion about which role you could take in a game, for example.

This is the context for using "sería":

Yo sería Superman [if I could, in my dreams...] This is expressing a desire; what you would like to be.

I hope this clarified it.

Un saludo

Inma