So, along with a new "final" boss (making it a 4 boss straight run if you play it right), there's a new ending and a new piano music that plays on the credits. Which I actually thought to be really good!
Sadly, it doesn't last for the whole credits, being followed by "Longing". But a new music for a new ending...Sounds demon's soul-ish, but I like it.
Enjoy!! ^^