Actually, I'd rather go back to the 1940's, listen to old music, look at art-deco bulidings, fight Team Nazi, and do battle with Adolf Hitler, Pokemon-style!
Either that, or just see the Poke-World 1960's and trip with college students on Dream Smoke, while stopping an anti-war rebellion.
But, of course, going back 2000 years sounds particularly awesome. What would be really awesome is to see some sort of ancestor to Giovanni and Silver be the main villain. He would round up the outcasts of ancient soceity, and they would be *dun dun dun* Team Rock.