If small people should ever become a kind that will appear in the One Piece world then the only ones I see coming would be people like the inhabitants of Lilliput in Jonathan Swifts "Gullivers Travels". I´d actually pay money to see Luffy fight a micro-sized antagonist.
Other then that, yeah, the monkey-men island is something I´m pretty sure about. Also if they get to machine island then there would be quite an amount of how I call them man-chines. With Man-chines, monkeys, giants, fishmen we already have enough different specimen of humanoids for a pirate manga. We even got aliens in a side story.
If Micro-People appear then it´s ok, maybe even harpies if Oda wants to pay some tribute to ancient story´s about treasure guardians but then it should be enough. This is not Digimon or Yu-Gi-Oh were every creature that fantastic or real that ever crossed the peoples mind has to be included into the story.
That sounds like a good idea those Lilliput. Maybe some sort of Faries or some other type of very little people will be introduced on an island. I can almost imagine them coming across that type of species and the hilarity that would ensue. With perhaps one of them having ate the Giant Giant no mi or something else. Aside from that I too am expecting some interesting characters from Mecha Island.
As for Aokoji being black I don't know why it matters so much what other people think his character is. I admit that before this thread I hadn't even considered him to be anything other than black if I had even thought about it before.
In regards to that picture where there is no in between I have to note two things. The first being that all of those characters have about the same skin tone which is about the same as my father's skin tone. The second being I thought that Wiper and the Shandians were supposed to be kind of like Native Americans, yet Wiper's skin tone is the same as evryone else in that picture.