When Ace was burned by Akainu, I started thinking. If magma is superior to fire, then what about the other logias? Taking into account the rarity of logia class DFs, I came up with a theory. Here it is:
There are six "elements" (categories) of logia type fruits. They are light, dark, air (wind/sky), fire, earth, and water. In each element, there are two logias (so far, there may be more), but each one is different. This sounds confusing, so here is an example:
Light: Pika Pika no Mi/Goro Goro no Mi
Dark: Yami Yami no Mi
Air: Moku Moku no Mi
Fire: Mera Mera no Mi/Akainu's Logia
Earth: Suna Suna no Mi
Water: Hie Hie no Mi/Numa Numa no Mi
I hope that makes things clearer. Ice is in with water, as it is water's solid form, and I threw swamp in there as well because it seems more liquid than anything. Both fruits fit into their category, but are slightly different than the other in its category.
The last part of this theory is that one fruit in each element is superior to the other. The easiest example of this is Ace and Akainu, but I thought that Kizaru would beat Enel, and Aokiji would defeat Caribou (just with DFs). This also means that there needs to be another earth, dark, and air logia. Any thoughts? Remember, this is just a theory, so I don't entirely believe myself.