Tan or dark tan would be a good replacement for the yellow. They are very close to yellow, but look more updated. Also, there could always be some yellow on decorative elements inside the castle.
I'd be most excited about getting updated versions of the knights:
I'd love to see this classic heraldry on updated pearl grey armor, with updated printing techniques. I'd also love to see a new version of the classic x105 visor. Of course, they'd have to have their squires, and the castle guards, and new horses. Maybe even a King and Queen.
Anyways, I doubt it will ever happen, but a massive, updated D2C of the classic 375 Castle would be amazing. I would definitely buy it.