On what level does "Harry Potter was based on a real boy" or "A Nigerian stranger needs you to help with the transfer of eighty million" sound plausible?
Well, not just here, but for most of your attitude to it in this thread: try again without using the circular logic of getting historical if you already assumed historical, or fictive if you already knew it's fictive. A criterion for historicity or plausibility which only gets you what you assumed to start with, that's begging the question, or in other words a canonical fallacy.
The criteria for why Jesus is historical
must be applicable to known historical
and fictive characters, and tell you which is which correctly. In fact, they must work blind, and preferably double-blind, if that test is to mean anything. If you read a story and didn't know if it's historical or not, a working set of criteria should be able to hint that without prior knowledge of what conclusion you expect.
If you can't apply it to known fictive characters without getting the wrong results, and need to rationalize why it's not applicable to those, that should be your indication that your "criteria" are just glorified circular logic.
So to return to the topic, why wouldn't it be plausible that a story would be based on a real boy? After all Milne's stories are based on a real boy and a real teddy bear, and Lewis Carroll's Alice novels are based on a real girl. Of course, they didn't actually do any of the stuff ascribed to them in the novels, but they're based on real people. So what would tip you to the fact that Harry Potter isn't based on a real boy, if you didn't already know it's fictive?
Again, try to determine that stuff without prior knowledge.
Or for that matter, how would you know that Poe's mad arab Abdul Al-Hazred isn't based on some Arab by some similar name he's heard of, and who was into ancient occult stuff, if you didn't have that prior knowledge? I mean, sure, the original name would probably be without the double "the" in the middle, and be something more like Abdul Hazred or Abd Al Hazred, meaning "servant (or slave) of Hazred." It's plausible. In fact, it's a common name construct in the Arab world, and medieval muslims were quite heavily into scholarly research of ancient manuscripts, astrology, alchemy and occasionally occult stuff. So it's actually very plausible as a possible origin. It's also false.
How would you know if Buratino isn't based on some real doll (but not a Real Doll

) or a lying and disobedient boy, if you didn't already know it's based on Pinocchio, which is itself a work of fiction?
Or if you had only the novels of Gatien de Courtilz de Sandras' and Alexandre Dumas, how would you know which deeds of d'Artagnan are real and which are fictional? I mean, sure, you can try to see which deeds are mentioned in both, or which get changed (as per the 'embarrassment criterion'). But ultimately the fact is that Dumas basically just wrote some fanfic based on the earlier author's novels, rather than any historical information. High quality fanfic that ended up a masterpiece in its own right, but ultimately still fanfic. Although the character is based on a real person all right, a lot of what would pass those criteria is actually fictive. And going by just what sounds plausible, even more ahistorical fiction in there is plausible.
You are correct in that plausibility can be quite subjective. For instance, someone might consider the faith healing explanation for a recovery at Lourdes to be plausible, while someone else might not.
But we already know that the same applied to a whole population is a fallacy, and means nothing at all. That's the argumentum ad populum right there. Why is then reducing it to a subjective personal criterion of one person worth anything at all, if even the averaged version of whole populations doesn't?
If we take plausibility to mean the ease with which you can hold a belief, then the number of questions that arise uniquely to that belief would be something of a benchmark as to how plausible it is. With the example of Harry Potter or the Nigerian 8million, there are quite a lot of pertinent questions associated with the beliefs, so I cannot believe you seriously consider them to be plausible.
This is still something of a subjective matter, but we at least are not totally in the dark.
But ultimately if there is an objective ease to holding a belief, that's measured in how many people believe it. Yet we also know that that's a BS fallacy, not something that actually makes that belief reality.
And to illustrate why it's bogus, consider this:
According to a poll,
three quarters of Brits weren't convinced that the Battle of Blenheim (a major British victory) was real. Over half believed that Nelson commanded the British forces at Waterloo. Nearly half believed that Sir William Wallace was fictive. Over half believed that the Battle Of The Bulge, a major WW2 event, is basically Hollywood fiction. Nearly half believed that Custer's last stand and the Battle of Little Big Horn, or for that matter the Hundred Years War, were entirely fictive. About
two thirds believed that king Ethelred the Unready was a fictive character. (And with a name like that, can you blame them?

)
And when you move to one-person beliefs, you can land in such categories as the 11% who believed that Hitler was a fictive character, or conversely the 5% who believed that Conan The Barbarian was a real historical person.
In another poll, some 58% of the Brit teens believed that Sherlock Holmes was a historical person and actually lived at 221B Baker Street. Conversely some 47% believed that king Richard the Lionheart as fictional. Some 47% believed Eleanor Rigby was a real person rather than a creation of The Beatles.
So, yeah, those are all very plausible scenarios. In fact, not just plausible, but believed by a heck of a lot of people. If you asked merely whether they find some of those plausible, rather than outright historical fact, you'd likely get even higher numbers there.
And again if you move to one-person beliefs, you risk falling in such categories as the 1-in-5 who believed that Sir Winston Churchill was a fictional character.
That is why 'plausible' doesn't mean jack squat. Personal ease of believing something historical or not, doesn't actually make something historical.