Tank Engine, but I agree somewhat
There's a tool in MS Paint, a rounded-square option, like the circle option, which I believe would be more beneficial to the concept art, and more believable as minifig heads.
Furthermore, perhaps it might benefit us the viewers to see how some of these look with masks on? They being Matoran and that typically being the custom in this universe.
I do like a few of the concepts rendered here. In particular the tan-faced, red-eyed one at the bottom-right of the top photo. It's more visually interesting above the others in my eyes, and I think with a little tweaking you could have a seriously nice character design there.