Sometimes people ask a rhetorical question, and deliver it as a sentence, without the upwards inflection at the end that we see in genuine questions. Not all rhetorical questions are delivered in this way, but it's an important enough difference in tone that when writing, the question mark is often replaced by a full stop.
That is how I used to write these statement-questions, but a friend objected, finding the full stop mentally jarring having just processed the words as a question. Did I misread? Did he mistype? And so I started putting a question mark after the full stop in these cases: "Why would anyone do this.?"
It's a solution that's just satisfying enough for me to keep using it – for people who get what I'm doing, it makes the statement-question unambiguous, and should hopefully reduce the mental jarring. But even assuming that people know what I'm doing, it is unsatisfactory: I've tried to internalise the full-stop-question-mark over a period of years, but that question mark still makes me start upwardly inflecting the end of the sentence that I've just typed specifically to not get upwardly inflected.
It occurred to me this evening that a better solution would be to borrow and then modify from Spanish. Upward-inflecting questions would get an inverted question mark ¿ at the start and a regular question mark at the end ? . As soon as the question starts, the reader would prepare to upwardly inflect at the end.
Statement-questions, on the other hand, would only get the final question mark.
Upwardly inflect: ¿Why would anyone do this?
Don't upwardly inflect: Why would anyone do this?
So, the final question mark merely confirms that gramatically, the words just written are a question. The presence or otherwise of the inverted question mark primes the reader for the appropriate inflection.
As practical suggestions go, this is somewhere beyond the "utterly useless" end of the spectrum that covers everything I've ever suggested before. It would require the internalisation of a different punctuation system by all English speakers, forgetting entirely the cues that come with the single question mark at the end (cues that would live on in all the centuries of books written before my idea becomes standard). But it seems an elegant solution, and I thought it was worth documenting, albeit in a post which I'll sneakily upload to LiveJournal in the dead of night Australian time and won't link to elsewhere.