Theory The "D" in the Name. Origin and Meaning
The crucial question arises: why the "D" in the name? My answer is: to indicate the origin! The country of ワ!! (Wa no Kuni).
Let's try to give substance to the theory.
Let's assume that the name of the ancient kingdom was 和ノ国 (wa no kuni, country of harmony in the Latin alphabet), and that in the 100 years of void, the name was erased by the government to make people forget the characteristic of that great country, harmony. It was then started to be called, by those who still knew the pronunciation, ワノ国 for the simple reason that the pronunciation is the same, except for a slight accent variation (in the Latin alphabet, wà no kuni), otherwise very much like Oda or very much like One Piece as a finding.
The descendants of its inhabitants, initially, to never forget the characteristic that has always distinguished their country of origin and its values, adopted the kanji ワ in the name, as a testament to their origin. Then, as IM himself states, over time, the meaning was lost, and it continued to be passed down as an inheritance, perhaps transformed into the similar symbol (a letter in the Latin alphabet, but in general graphic symbolism, a smile, a crescent moon, anything) D, probably because any reference to the ancient kingdom had become risky. Another silly idea, but always very much like Oda and very much like One Piece.
The dream before Joy Boy, then Roger, and today Luffy's dream would be to create the country of harmony, 和ノ国, the country of the D. (P.S. the kanji 和 indicates Japan. The moral of the manga, opening Wa no Kuni, discovering its name and its secret means recreating the country of harmony. The Japan of harmony is the open Japan).
Not to mention the fact, but in truth, we are talking about it anyway, that Joy .D. Boy (with the kanji ワ instead of the D symbol) written in kanji ジョイ ワボーイ (Joi wabōi) sounds very similar to 調和ボーイ (Chōwa bōi), meaning 'harmony boy'. This is linked to the fact, in my opinion, that since the kanji 和 has been banned, even in the name, it has been replaced with ワ, transforming it into 調ワボーイ (so it lost its meaning, that is, it becomes 'music boy', Binks' sake?? just kidding), and over time, being remembered as a bearer of harmony, it has become ジョイ .ワ. ボーイ, Joy .D. Boy.
This reflection arises from the fact that, perhaps due to my ignorance, I cannot find a reason why what could be translated into any language as the Country of Wa could find the original correspondence in Japanese as 倭ノ国, or even 和ノ国, with the first kanji in both cases clearly referring to Japan at the level of meaning of the ideogram, as is the Country of Wa in the manga. Instead, a kanji 'ワ' that sounds the same but means nothing by itself has been used, preferred to its pronunciation cousin, even though it carries thousands of years of history. Clearly, there is the possibility that it is a mere stylistic choice, to hide the reference to Japan behind a veil, a play on words (and sounds). But there is also the possibility that it is tied to plot logic, and I have chosen to go down this path.
I realize that dozens may have noticed this connection before me, and probably if it is not talked about, it is because it has been discarded, but having missed this passage, I would like to understand why and where my reasoning loses its way.
Following a quick check, it seems that the kanji 和 would really have been removed from the world of One Piece. It is never mentioned even though it would be required several times for words that require the phoneme WA or for words or phrases that could alternatively be composed with the kanji 和. Only in two exceptional cases, which still refer to things related to the country of Wa, is it used. To indicate the Wado Ichimonji (和道一文字) (although the construct to create Wado could not be different, I believe) and in the name of Kozuki Hiyory (光月 日和).
To top it off: Let's take One Piece written in katakana ワンピース (wan pisu) if we perform the usual substitution, we get 和ンピース (wa npisu), which means JAPANESE PEACE!
Goodbye.