i got a few ideas, if u care... :D
1. I saw a youtube vid and it showed that there was an unused enemy in the game's coding (or whatever...), Blargg. Like the one from Yoshi's Story, if u played that. I don't remember what tool you use to put it in the game, but could the Blargg possibly be in it (sry if I'm asking too much...)? It could replace the Bully (small one), with a different ability, like it could blow fire at you, and you have to grab it and throw it into the lava to kill it. again, sry if i'm asking too much. :(
2. Remember that green koopa you jump on to steal his shell? Maybe he could be red, or a Dry Bones?
3. Have you thought about making it a little Luigi's Mansion-ish? Maybe the castle could be Luigi's mansion from the game of the same name? And the path to the castle could be Boo Woods like in said game? Courtyard could possibly be Graveyard where Bogmire was fought.
sry to say so much, i don't want to be demanding, they're just ideas.