Wily's castle is usually a rehash of the older bosses fought in succession. Instead of that, you've already faced the heroes, so the castle should contain the villians!
This is my idea:
- Cave Story - Balrog/Misery (obvious)
- Iji -the Tasen general (I think, the big boss)
- La-Mulana - the giant boss from mausoleum (I didn't get far, are there better ones?)
- Spelunky - Olmec (with similar idea to the real fight)
- the Underside - Johnny
- Chzo Mythos - John Defoe and/or son
Runman, again I have no idea.
Still need like two more bosses though, 8 super-robots.