In a very bad film of Spanish origin and a "Comedy" known as "Los Japón" the premise is postulated that in 2019, in the absence of a better (and male) heir available to the Chrysanthemum Throne, the next Emperor of Japan be...
...a Spaniard from Seville chosen apparently at random. (The "explanation" is that the guy is the last living male descendant of an Imperial prince who traveled with the Keicho Expedition and settled in Spain.)
Of course, even though this would essentially be equivalent to Prince Wu's appointment as Earth King in LOK, this silly plan goes ahead anyway. The only thing that prevents it is that at the last moment the guy decides that being emperor is not for him.
As a movie it is worse than hitting a mother, but as an example of implicit alternative history it serves.
Although personally I am inclined to think that what would have actually happened is something like this:
*Palace of the Government of Japan, extraordinary meeting of the Council of Ministers, shortly after the death of the Emperor*
Prime Minister: "Gentlemen, the situation is critical. Our beloved Emperor has died and we have very few options available to succeed him. Right now our options are to name the Princess as future Empress... or choose one of the two proposals that the ministers of Finance and Foreign Affairs have to present to us".
Minister of Finance: "That's right, sir. My proposal is to take advantage of history. As you undoubtedly know, 600 years ago one of our imperial princes, who had converted to Catholicism, traveled with that expedition that we sent to Spain. The prince "He stayed there and had a family. We have located his descendants."
*Mutters of disgust among those present*
Foreign Minister: "Let's see if I understand you. Are you seriously proposing that we appoint as the new Emperor a gaijin who has never lived in Japan in his life, probably doesn't know even half a word of Japanese, and whose only merit to rise to the throne is that one of their ancestors 400 years ago was an Imperial prince?"
"Yes, that's right! That's how it is"
"That's complete nonsense and I refuse to condone such nonsense. I actually don't know why we're even considering it."
"Well, what's YOUR proposal?"
"Simple. Do the same thing that all the great powers have done. Forget about the Throne and the Emperor. Proclaim a presidential Republic, as the Americans, the Germans, and the French have done before us. I volunteer to lead the transition to a Republic of Japan..."
*Mutters of disgust even more intense than before*
Prime Minister: "Okay gentlemen, it seems that there is no consensus. But we have to make a decision."
Minister of Culture: "Prime Minister, I would like to present my own proposal."
"Go ahead."
"I was looking through history and found the interesting fact that the Spanish themselves faced a problem very similar to the death of Ferdinand VII: a female heir and laws that prevented her from inheriting. The solution they adopted was... ingenious. "
"What did they do?"
"The king enacted an amendment to the Law of Succession that allowed his daughter to inherit. The Emperor's will has not yet been made public, has it?"
"No..."
"Well, what if the Emperor had expressed his desire for his closest female relative to inherit?"
Minister of Finance: "I can't believe we are considering this. Think of the immense damage it will do to our economy if this is done. I think I will fully back the Minister of Foreign Affairs on this matter and support his proposal for a Republic "
*Muttered debate for almost an hour.*
Prime Minister: "Then I think we are all in agreement that the Minister of Culture's proposal to modify the Succession Law so that the Princess inherits is unanimously approved."
"That's right, Prime Minister."
"Let's get to work immediately. I want you to dig up all the precedents that support the idea that an Empress will not result in a disaster. The negotiations with the rest of the Diet parties will be tough, but it will be worth it. And as for those Spaniards... the less them know about this matter, the better. Do you understand?"
"Yes, minister."