Below is a simple sketch of what my potential drawing device will look like. It will consist of a servo with two marking utensils on it, a DC motor, a roll of paper and two analog sensors to be named. The DC motor and servo will each be connected and controlled by different analog sensors. One sensor’s readings will control movement of the DC motor while the other analog sensor will control the movement of the servo based of the information it picks up. I believe that having both the motor controlling where on the roll marks will be made and the servo controlling how the marks will be made will result in some interesting patterns/drawings.
Below is another potential drawing device I have considered pursuing. This device will contain a DC motor, servo with marking utensils on it, and two different analog sensors as well, however it will be marking small square cardboard canvases. It will work in the same way that the first device would in that the motor and servo would be controlled by different analog sensors to control the markings. The DC motor would spin the cardboard canvas while the servo would fluctuate based on the sensors readings.
The commonalities between the two devices is the use of two analog sensors that drive the DC motor and servo independently. I believe that having two different components using different information to control different components will result in interesting compositions.
Both these ideas look solid. The first would give you more range of mark making. If it's mounted vertically, then you could use the tension of gravity to keep the paper taut – I think that is your main mechanical challenge.