If you truly want a tablet rather than an Ultrabook or laptop, you want portability and you want it to work with the R200, a prime candidate would be the HP Spectre X2, which has an early 2016 model with an R200 built in.
An updated version of the Spectre X2 has also been released.
You should be careful when considering older tablets that claim to have RealSense built in. Some of them use an older version of the camera called the Snapshot, or R100, which is focused on photo-taking and editing and does not have the full capabilities of the R200. So it is important to include the word 'R200' in your search terms.
If you are willing to use a USB cabled external R200 with a PC Ultrabook with Windows 10, an Intel Core processor and USB 3.0 (maybe one with a detachable screen that acts like a tablet) then the number of available options would increase significantly.
I would caution that developing a prototype with the R200 may be a dead-end if you want to then turn the prototype into a commercial product, since others would have to have an R200 camera to play it. A good avenue for you may be to consider converting for your prototype for the forthcoming Intel Project Alloy headset, which contains a ZR300 camera in the present pre-release models.
You may also find my list of guides for developing with RealSense in Unity useful.
Thank you so much for such a detailed answer! I decided to take your advice on steering clear of devices with realsense built in. (That and I have had bad luck with HP as a brand) Should anyone drop by and read this thread in the future, I ended up purchasing this:
After some tests with my r200 I can say I am very pleased with its performance. If you need a 2 in 1 I havent found anything better.
Again, thank you so much MartyG for your time, you helped me out a great deal!