It would probably be based on the Japanese elements. Smoke is a form of air, Magma is a form of fire. There are pure element fruits such as fire, like Ace, but the others are just sub-categories.
The reason poison and wax aren't Logias is probably because they have to be derived from multiple sources. So I guess Logias have to be "pure" in a sense.
Further reading on this can be found:
https://en.wikipedia.org/wiki/Five_elements_%28Japanese_philosophy%29
We already discussed this on irc, but yea that actually helps me conceptualize it a bit better. Thanks for the link.
ロギア (logia) is a special katakana reading used on the kanji 自然系, which would normally be read "Shizen Kei" meaning "Nature type."
Zoan would normally be read "Doubutsu Kei" or "Beast type"
Paramecia would normally be read "Chou-Jin" and mean "Super Human"
But basically, based on kanji, it's not elements or anything like that-- any raw substance of nature could be a Logia (hell, there's a "swamp" logia). I'm not sure what the reasoning is exactly, but my guess is that Rubber is a refined material, while poison is a substance created by plants and animals, which is why they're not naturally existing raw substances.
Aaaaaaaand I know I can always count you for the clarification :P
Yea, that makes a lot of sense, and makes all the logia consistent as well, as long as you assume Caribou's Swamp Swamp was referring to the general "muddy" nature of swamps and not the geographical area known as a swamp (because otherwise lolol Tundra Tundra and Prairie Prairie logias...no)
I think it's also important that a Logia-user can turn his/her body into the respective element/force of nature.
For example, Magellan/Mr. 3 could "produce" poison/wax, but if I remember correctly, they couldn't turn into said thing.
Luffy probably doesn't count because it's a permament thing.
Magellan and Mr.3 could both turn into their respective fruits. The key points Chou brought up (elsewhere) regarding Luffy is that he is solid AND he can't turn it off, so you're probably mostly on the ball regarding the permanence thing.
So just for fun...what Logia are left?
(chou please correct any japanese issues obv)
1.) Rock Rock (Iwa Iwa no Mi)
2.) Water Water (Mizu Mizu no Mi)
3.) Blood Blood (Chi Chi no Mi)
4.) Plant Plant (shokubutsu is what I'm getting but that's long lol)
5.) Air Air (Kuuki Kuuki no Mi)
6.) Wind Wind (combine 5 and 6 if too similar) (Kaze Kaze no Mi)
7.) Oil Oil (Sekiyu Sekiyu no Mi)
8.) Wood Wood (Moku Moku no Mi, from mokuton)
9.) Cloud Cloud (Kumo Kumo no Mi)
10.) Metal Metal (I was getting Kane, but I thought that was more bell, so tetsu tetsu? I know that is iron or steel though)
11.) Steam Steam (Jouki Jouki no Mi)
12.) Clay Clay (Nendo Nendo no Mi)
What other potential logia are left? I figure, with Haki so emphasized now, we're going to get quite a few more Logia from now till the end game, so should be interesting to make some predictions based on that.