My vote for the last band would either be an original/Pandemonium era reunion of Celtic Frost (but T.G. would probably want more money than all the other bands combined), or Diamond Head; or, with Maiden taking a break, maybe Bruce Dickinson's solo band (with Adrian, of course).
Now that I've gotten my "wish list" out of the way, since it seems we're probably searching for a band a little "lesser-known" for the final slot, I've still got to lean towards Therion...