It would not have been Spiderman.
Don't get me wrong, I like Spiderman. It's just I don't see spiderman being very musical. I don't seem to be alone in that, since the folks doing it have been working more on the angle of spectacle. Nope for my money the best thing musically associated with Spiderman is the Ramones cover of the old themesong.
Now what I would have done is Ranma 1/2. Now some of you fine fine people might not be familiar with the property, but trust me it lends itself to light musical comedy. It is the story of Ranma, just a poor guy who wants to be good at the martial arts. Unfortunately, his father has wrecked his life in two ways. The first is that he has arranged a marriage for Ranma with another school of martial arts. Ranma now must go live there with his dad and his new family. Maybe this wouldn't have been so bad except that on they way Ranma fell into a cursed puddle. Because of this whenever he gets wet he turns into a girl. He stays a girl until he can find some hot water. From here the show followed his betrothal to Akane who claims to hate all boys, and the various love complications that enter into their lives.
As funny as this was on TV, I can just imagine how great it would be on stage. It would be easy enough to use the same tricks magician use to switch the actors who'd play boy Ranma and girl Ranma. There would be singing, martial arts, farce. It would be great.