As we already knew, two giant companies, Apple and Google, finished the last Christmas shopping. Google bought several robotics, sensor related companies and Apple acquired PrimeScene, the patent holder for Microsoft Kinect.
I found funny add-on type accessory for iPad, Structure Sensor.
Have a look the below video and you can get an idea of it.
Now it is quite expensive, almost 4/5 price of a iPad, $349 but the cost might be suitable if it will be integrated into the new iPad version.