For myself:
1. I just beat Olrox today, and man that's a fun fight. I would love to see it return even "as is" in another game.
2. I think one boss that's "due" for a new game is the Hippogryph, considering how it's soul has been in two games for giving a "rocket jump". It needs to be beefed up quite a bit though. Actually, I consider this a "remake" of the Mr. Chicken boss from the two Kid Dracula games, so it might not count as an enemy that's only been in one game.
3. I gotta go with Venus Weed. The regular enemy is quite difficult to deal with, a boss could be murder.