Genesect, the current end of the National Dex, is #649.
The three stages of the next starters will add nine, with Froakie's final form being #658 (assuming they don't do another Victini thing)
So, #666 will be just 8 pokemon ahead of the starters. Therefore, #666 will probably just be early Route 1 bird/rodent/bug fodder. Hell, it'll probably be Pancham's evolution.
