I am really enjoying C3. One thing that I think would be fantastic to add is the ability to do pixel art on an tablet and have it update on your computer. This would really make the workflow of C3 awesome.
I found the pixel editor to be still pretty good a little laggy. The reason you would want to use a tablet is to use a stylus pencil to do the art. It's much easier to make pixel art that way instead of using a mouse. It's also more fun.
I made some other suggestions to the pixel art editor, but if I could add one more suggestion. Adding in the undo and redo as a button so that on a tablet you can undo your mistakes would be really helpful.