I'd prefer it if they kept the earrings and bracelet as they are and add a necklace, brooch and tiara to them in order to create a full set.

There should still be 5 diamond sunbursts from the top part of the Van Kempen tiara. They already were on a frame with 4 pearls. I think those pearls ended up in the Antique or Pavlovnik Pearl tiara. However they could be replaced by drop shaped turquoises and the central diamonds of the sunbursts could also become turquoises. That would make for a suitable historic piece to go with Beatrix' earrings and bracelet. A decent necklace of turquoise stones set in smaller diamonds isn't cheap but still within the financial reach of the family.
I would also love to see them add an amethyst and pink topaz set to their collection as well as a new smaller sapphire tiara so the necklace of Queen Emma can be restored to it's original shape.