Thanks for the feedback! We have pretty different views on rank mechanics in general, since I think that they allow for more dynamic dialogue between the game and the player, and a lot of games I love use rank and use it really well, like Blue Revolver, ZeroRanger and Godhand. Something like extra spells in Imperishable Night could be viewed as a similar mechanic in the context that you are describing. It basically rewards you with more game for better performance.
I do agree that I can still improve on the way these mechanics are conveyed and contextualized though. Ideally I would like to make it so you can feel the opposite - finding out that there is still more to the game than you initially thought. Additional enemy waves from quick kills serve the same purpose.