Let's talk about Logia:
After thinking about it a long time, I finaly realized why Pica and Trebol aren't Logia (from a story standpoint). If Pica was a Logia then his fight with Zoro would have ended quickly and bloodyly, as how Vergo put it, that expanding your body against a haki user as a logia is just a weakness, and how Trebol showed how to create an oppening with the asumptions that the oponent makes during the fight.
So: First observation: What distinction do you think that oda make to say "this is a logia"? I mean under the rules there should be an earth or stone logia, but he chose Pica to be the "stone asimilation man". There hasn't been a logia for a man made thing, but the "logia-lite" paramecia have some nature link, wax is an animal fat byproduct (I think), soap as well, poisons are common in nature, as well as sticky substances with flamable properties, and while both light and darkness are logia, shadow is just a paramecia.
My personal guess is for pre-timeskip enemies was to not to make them too strong for the crew to beat, while the post timeskip the contrary, to not make them too easy to defeat. Funny how that works.
2th Observations: Weird Logias (other than the darkness fruit):
Caribou's swamp that can absorb things, and store them without affecting his shape (and weight?).
Caesar's gas fruit that gives him control over.. All gases? That's unconfortably general for a fruit, I mean can we expect a solid and liquid fruit? My hope is that he just absorb gases (like propane for the flame sword, helium for the baloon, and whatever poison used against luffy) to use in the future, just like he absorbed the gas weapon during the final part of the fight with Luffy, rather than being keept in check by just his own weakness, rather than strenghtened by his cunning.
I feel like I'm being too nitpicky about this, but I don't like devil fruits with too broad or too unconnected powers, main culprits being the Love fruit, the OP fruit, the Hobby fruit and the Paw Fruit. With the shadow fruit, the ghost fruit, and the two mentioned weird logias being a second tier.
3rd and final question: the Hypotethical Water Logia, if it were to exist would it be the only devil fruit that bypasses the water weakness or the weakest devil fruit because whenever it's used it exhaust it's user?