I'm starting to think of Harmony of Despair as an updated Vampire Killer-style game, in much the same way Symphony of the Night could be considered a successor to Simon's Quest. Basically, if I said "let's make a new Castlevania game inspired by Vampire Killer", I think the result would be very similar to Harmony of Despair. Here's some main similarities:
* You're main goal is to defeat the boss of a stage, but in order to get to the boss, you have to first figure out how to navigate the map properly. This may involve a lot of backtracking, caused by getting keys in Vampire Killer or activating "switches" in HD. Backtracking is possible like in many other games, but all self-contained in a single "board-like" and "maze-like" stage.
* A sub-goal is to find items in chests scattered throughout the stages.
* They're both rearrangements of earlier Castlevania titles. Coincidentally, they both have remixes of the original Castlevania game and Simon Belmont.
Screen zooming and online multiplayer options could be considered major advancements to the Vampire Killer formula.
Of course, they're not billing this is a successor to the obscure Vampire Killer game like Castlevania: The Adventure ReBirth was touted as a Castlevania: The Adventure upgrade. But I wonder how much of it was inspired by it and intentional.
All the more reason for me to want them to re-release Vampire Killer one of these days.