This game looks jawesome! Way too many great fangames...
As for boss suggestions, there was one incubus character in the radio drama NitM thing and I totally forget his name, but maybe you can stick him in this game somehow? He had some kinda confrontation with Maria so it might work as an edited Succubus fight.
Some other ideas:
-Stained Glass Knight (obviously of much higher caliber than Haunted Castle one)
-Since it's in Serbia (right?), would Baba Yaga fit in somewhere? Maybe as a merchant character?
-Other folklore from around that country would make great enemy ideas, I think.
-Aaand just for the hell of it, some souped up enemies from any of the Gameboy CVs would be cool.
EDIT: Holy shit, if the game is in Serbia, you guys have a veritable cornucopia of Russian mythology to work with. (It's under Slavic mythology, but it's a close region I think)
http://en.wikipedia.org/wiki/Slavic_mythologyKatschei the Immortal/ Koschei the Deathless is a lecherous... sorceror? Lich? Anyway, he did lots of black magic stuff to be immortal and would be a creepy character for Maria to fight.
See also Rusalka, Kikimora, anything in there.