It looks like you are using Coreldraw for the lettering wich is probably not the best for lettering and coloring. The colors doesnt work well here in my opinion(is it maybe unfinished?) The pencilling could also made to look like inking by adjusting "curves and contrast" in a program such as Photoshop. The minimallistic appraoch also worked better for me. Detail could be added while being minimalist by drawing large areas with detail(such as grass or clothing patterns) A good idea is also to have detail against flatness(like the way you would use darkness against light for contrast)
M
