The VG Resource

Wouldn't a Castlevania MMO actually work pretty well?

No, not really. While Maple Story has a working 2D RPG format that incorporates multiplayer; it doesn't, um, rely so much on certain skills that get you by in Castlevania.

There is however an XBLA game that's Vania with multiplayer Co-Op in a mini-dracula castle. That seems to work ok.