While Maria does not have children, she has two nieces, Karina and Elisabeth-Maria, who may be long term prospects. Given the way, Karina's brother was treated, she may not be overly keen, which may leave Elisabeth-Maria.
If King Michael's headship of the family is to be inherited by two of his daughters, I see no problem in the long term if it is one of his granddaughters who takes on the mantle of the dynasty.