I think we must forget any restoration. This Swiss-born British-national will never become King of Romania. The former royal families of Portugal, Romania, France, Germany, etc. are still treated with égards, given a place of honour, are received by the authorities and well-wished by the public. That is what they have.
They still have wonderful historic assets like the
Burg Hohenzollern (managed by the Hohenzollern family foundation), the
Château d'Amboise (managed by the Orléans family foundation),
Castelul Peleş (given back to the former Romanian royal family) and that is it. They will be prominent members of society, living embodiments of a great history and more it will not be. I do not see that Bulgaria, Romania, Serbia, Portugal suddenly change their republican constitutions.
Note that Greece, in the deepest turmoil of their existence, faced with the most deep economic crisis ever and the core existence of the state rocking on its foundations, there was no any sound to go back to "the good days" (the monarchy). Not even when all politicians were lashed and punished by the electorate, there was not even the most remote call for a return to the monarchy. So if even in such circumstances, the democratically elected polliticians and the republican state só malfunctioning, confronted with só much mistrust, there is no call for a monarchy... we can safely forget any other state to become a monarchy again.