I think there were some really good bosses in PoR. Keremet comes to mind and the PoR version of Dullahan was awesome.
I'd like it where you have to fulfill certain conditions to beat the boss like (and I'm only using this as an example because I can't think of a better one) God Of War. Maybe in order to defeat a certain boss, a vampire for example, you have to beat him with the Cross or soemthing like that. It'd put a new twist on boss fights and make them a little more strategic.
Hmm.. CV64 comes to mind... Vampire bosses that were about twice as strong during night time and pretty weak during day time?... Nah I MUST be mistaken, it can't be that good now can it
As for new and original bosses.. I'd like a return of orlox only then enhanced a bit with a diffirent skillset instead of a drac copy, also I was thinking what if death appears in more than one form? like instead of his grim reaper appearance there a numerous other
gods of death from diffirent cultures, those seem untouched by castlevania. Those could show up as bosses.
To sum it all up, I don't have many great idea's for diffirent bosses but I do agree the bosses can be made more tactical and challenging so that it doesn't become a simple hack and slash fest.