Aeon Blue - those are great; very appealing. If you were to vary your line weight a bit (fatter on soft edges and round forms, thinner on hard edges and angular forms, etc.), I think it would instantly improve them by about 60%. Good use of line-weight can add as much to a drawing as color can.
Some random sketches: tbh, these are only tangentially related to DF; I did them for a contest on CGHub. But I think they'd fit in the DF world.
Also, a process shot!