I think we can count out a Greek royal, I doubt they're as close as they used to be. Look who went to the recent Greek wedding for instance.
What Greek royal married recently? Of Con's kids, the last one who is wed married almost 3 years ago.
Will is godfather to Pavlos' child. The Brits have always been close to the Greeks, and of course are related. I think if any foreign royals are included, it will be Pavlos or one of his siblings.
Other than their wedding they have only met foreign royals at things like the Olympics and one trip to Denmark. The Wessexes are the only ones who seem to attend weddings and such on the continent. Edward is godfather to Nikolai, but the others don't have any foreign godchildren. And none of them have foreign godparents, except those who are related to them. And because of marrying future Brits the last generations, there are not any closely related foreign royal relatives.
Charles- Prince George of Greece (his granduncle) and the King of Norway (he was married to the queen's Great Aunt, Maude)
Anne- had Prince Philip's mother and sister Marguerita
Andrew- no foreigns
Edward- Louis Prince of Hesse (not family, but brother in law of Prince Philip's sister Cecile, he raised her daughter Joanna after Cecile and her husband died, until Joanna died)And Prince Philip's sister Sophie
this generation:
Will: Constantine (a cousin)
none of the others have a foreign royal, even a related one.
I think we will have a mix of family and friends. I don't rule out an older generation, as it seems common enough.
I see:
Harry
Pippa
Pavlos
Hugh van Cutsem (he and Pavlos as Will is godfather to their kids)
Zara or Peter
and an older family member like Sophie, or even one of Diana's sisters