My rivals are usually references to random characters, usually from literature. My rival in Pokemon Blue was called Holden (from The Catcher in the Rye), and my rival in Pearl is called Owen (from Just Listen by Sarah Dessen).
I changed things up sometimes. For example, my HG rival is going to be called Shawn. Shawn is a form of the name John, and the name Giovanni is also a form of the name John. See what I did there? I guess it's sort of a reference as well, but I like to keep some meaning with my names, even for references to literature.