I can see the difference between the coloring and theme for each of your creative characters, but the lineart is pretty much the same just in different colors. I mean they could belong to the same world, seeing them lined up like that.
So what I will recommend you to do is to find artwork of other artists you like, study them and try to copy them. There are many aspects you could try to copy, like the build, lineart, color style or shading. I'm sure there will be more variety then!