Hard to say without knowing precisely what you intended to write
- The Subjekt is "ich", so the declension you want is "bin" instead of "sein" - I don't think "werde" would typically be used like that in German. It might be technically valid but I don't think it's traditional
"aber vielleicht bin ich morgen ein Kätzchen" is my guess? But (a) I'm also a learner and (b) I don't know precisely how you wanted to phrase it