For the French state visit to the UK, I am setting out some of my "hopes":
Queen Camilla: July can be quite warm, so I think we will might see the Brazilian aquamarine necklace. That could be paired with either the aqua tiara or the Greville Boucheron tiara. Alternatively, if she chooses to wear all diamonds, she could wear the Greville Boucheron tiara, and pair it with the Festoon necklace.
Catherine: Sapphire bandeau paired with a suitable sapphire and diamond necklace (Dubai sapphires?)
Sophie: Papyrus / Lotus Flower tiara, paired with a diamond necklace from the vaults