Working with hand-drawn animation in After Effects/Adobe CC (creative replies welcome)

Your drawings need better-defined edges and no gaps in the important lines. From your two images, I was able to get these two masks in Photoshop in under a minute an image by using the Select and Mask tool. They are pretty close. You can easily create a Photoshop action that loads the next image and opens the tool. A couple of clicks and then a save and close action and you are on to the next image. That's the best I can recommend with this artwork.  

You might also have some better luck if you use Lightroom to increase the contrast of the outlines and maybe increase saturation so that the Select and mask took has more information to work with.

A few more minutes with each image would improve the result. Both screenshots are the first pass at creating the mask.

The problem with Rotobrush for something like this is that your lines can easily move too far between frames and the lack of background detail makes it impossible to calculate the difference between frames. Pencil sketches are going to be harder to work with than illustrations that have been drawn with a pen. Traditionally, hand-drawn animation is created by tracing over the pencil on paper sketches with pen and ink on transparent acetate animation cells. The process is called Ink and Paint. If you are going to stick with paper, you are still going to have to do some painting by hand to eliminate the background. You might be able to use Photoshop to speed up the process, but most hand-drawn animation that is pencil or ink drawings on white paper is not going to be able to be automatically processed. 

If we saw a couple of your drawings (cells) we might be able to give you some suggestions that would speed up the process.

Since you already draw this on paper, so why you don't remove the background on photoshop and save it as PSD then imported to AE ready with transparent background