It's a bit rushed, especially since Koya-san is best done as an overnight trip--staying in a temple is the essence of the experience. Remember also that you will be jet-lagged on the first day unless you're coming from Australia or New Zealand.

Some people love Osaka, but I'm of the opinion that it's another big city with attractions that are not immediately obvious to the first-time visitor. Kyoto, however, has a different vibe than Tokyo, as well as more of what visitors think of when they think of Old Japan, although it is otherwise a modern city.

If you have a JR Pass, Hiroshima could be a very hurried day trip out of Kyoto on the Shinkansen, but Tokyo and Kyoto alone, with maybe an overnight in the Hakone area on the way between them, would give you plenty to do in eight days.