I would highly recommend any and all of Christian Eichhorn's Mörk Borg supplements - they all have fantastic layout and art, creative content that adds new systems and creative twists on existing ones, and capture the brutal and beautiful feel of the core book.
Hallowed is a great pick-up if you're looking for for a grand adventure also paired with plenty of general gameplay supplements. Hallowed's campaign ("Bone Heart Crusaders") takes you through the Valley of the Unfortunate Undead, with tons of locations, enemies, NPCs, and new systems - a base 'war camp' and threat level system being standout. Hallowed also includes many general gameplay supplements, like new classes, blessings, relics, D88 Crusaders (NPCs), and 2x d66 'Reclamations' (major and minor treasures).
Enjoy Hallowed? Check out Purgatory for even more unique gamplay supplements/additions. Want something to complement Hallowed? Check out Galgenbeck Sacrifice or Bergen Chrypt for a more campaign-focused adventure, or Apocrypha for something more focused on supplementing your existing campaigns.