Hi…Mohammad…
Do you have Burne Hogarths…DYNAMIC LIGHT AND SHADE,…If not, try to get it…That book will teach you how to take your figures to a higher level…atmosphere, and a whole lot of different kinds of lighting and shading effects that help to creat and enhance the feeling and mood of your figures can be found in that book…one of my favorites, and a must have book for anyone who is going to creat figures and put them into creative envirorments also…
Have you tried to creat any figures using CONTI CRAYON, or CHARCOAL. I think you might really like the feeling and effect that those two techniques produce in the figures…they will give your figures wieght, volume and mass, where you can really make the figures pop in and out of the canvas, more so than you can with just line…they also give you a better sense of atmosphere in and around the figures.
Just some thoughts that might be of help to you in your 2d adventure…
…Looks like you have a pretty good handle on 3d already.
I really like that mechanical horse that you created, and also your 3D toe studies that I just saw in one of your other threads…FANTASTIC… That castle with the bridge is also GREAT…
REALLY LOOKING FORWARD TO SEEING YOU TAKE YOUR 2D WORK TO THE LEVEL OF YOUR 3D WORK…WILL BE A GREAT SIGHT TO SEE 
THANK YOU ALSO Mohammad FOR THE VISIT, and the KIND WORDS THAT YOU LEFT BEHIND YOU IN MY THREAD…
TAKE CARE
Glenn