Nothing. Tablets are effectively dead in the CAD world. Go to a gaming hardware store/online store, look at their options for mice, off-hand keypads, and the like. Plenty of multi-button devices are available. Yes, you won't have a physical layout of "buttons" to click but with the Ribbon, programmable keystrokes, etc. the only thing you will be losing is the manual tracing option. Even that can be done via Raster Design and other tools.