I think that this is a bit overdue...but Devil Fruits shouldn't be limited to a rank/required to be bought. Devil Fruits aren't always sitting in stores, and if someone implements them reasonably into their history, that should be the only requirement for having one. If you want to limit the people using devil fruits, you could come up with a better way than grinding. Devil Fruits have lots of weaknesses, especially since most of the world is ocean. If you see someone being irresponsible, not playing out their weaknesses correctly, then they get their fruit taken away. Bam. Also, devil fruits need to be given a strict moderator. I actually volunteer for this task, since I'm probably the only hard-ass on the site.
Now, about Devils and Elementalists. God...when Leo told me about this one, I knew I'd have some qualms with it. Anyways, my personal gripes aside, they aren't even canonical at all, and they aren't even creative ideas. With the right devil fruit, anyone can be an elementalist, and having one brings up the question of what if a logia user of their element fights them, and what are the limitations? For Devils...do I really need to say anything? They are...well, a race that are created when a family of devil fruit users have multiple children. This brings up a lot of problems. Devil Fruits aren't exactly the rarest things in the world, but I mean, c'mon, they shouldn't be so easy to get that there would be whole FAMILIES of devil fruit users. Devil Fruits don't even get passed down genetically at all, so I'm not sure how that even works. Plus, when a family member would die of old age, the fruit would bloom again somewhere else, proving that devil fruits are separate from the person, even if their power is able to be used. So in short, I believe that these two archetypes, Devil and Elementalists, should be removed or at least kept for a later date, maybe a plot device.
And before I end this off, I want to say that I'd like to be an Admin, or at least a Mod. This site still needs a lot of help, and I'd like to put my experience in so that instead of it flopping without me, it'd flop with me. But I'll make sure it doesn't. Flop, that is.