Actually with her earrings Mette-Marit has played around quite a bit, but she sems to be an earrings-kind-of-woman, and not really interested in other pieces very much.
Yes Queen Mauds diamond tiara has the possibility of beeing a grand version of Princess Sofias tiara. We already know it has diamonds and has had turqouises in the past, which the NRF has alot of, and it could probably easily be adapted with more affordable stones like aquamarines (perfect for Mette-Marits colors) and topazes, and probably they can wear it like the base of the 9-province tiara in Belgium as well.
What they are waiting for with this tiara I dont know. Bring it out for the danish state visit, afterall QM2 herself has said jewels are ment to be used and not in a museum, and this tiara is not even in a museum, but in a box in a vault.