There are two things which will add even more realism. (not sure if I mentioned one of these previously)
1) I notice that when rotating the viewpoint of the game around with my finger that you have a light source and this reflects off the surface at different angles depending on the view, it certainly adds a lot of realism.
What if you were able to access the gyro sensor of the phone/tablet in play mode so that adjusting the physical tilt of the phone affected the lightsource direction and hence gave you some simulated changes in lighting as you tilt your phone up/down/left/right. The full screen view would still be locked into place but just the illumination would change and trick your brain into seeing a bit more 3D depth.
2) I've seen some apps on phones such a labyrinth, pinball etc. which fill the screen but adjust the view angle in realtime depending on the tilt detection of the gyro sensor. If you look carefully at the following video you will see what I mean, notice how the viewpoint changes very slightly when they tilt the phone. So when you tilt the phone to the right you would see slightly more of the right side of the buttons for instance. Tilting the phone upwards would allow you to see slightly more of the top edge of the buttons. Perhaps getting a bit complex but something to consider for maximum realism (with ability to turn the feature off). For example, see