Perhaps rolling backgrounds/stages? Could be interesting to see environmental changes and a "journey" through different locations to fight different enemy types. Essentially a campaign mode with world bosses. Could supplement the Knight enemies for "limbs" or components to a bigger Boss enemy.
All in all, the game play loop/concept is solid so adding complexity to that system with a story and levels would be interesting for my personal preferences.
That being said, I'm just an internet stranger so take my thoughts with a heavy grain of salt! Hahaha!