The 12.9 is kind of crap for gaming compared to the 9.7 ipad pro. Its very hard to hold, and use touch input, unless controls are center right and left.
What would be great is if there is more mfi support for the smart keyboard, or apple tv remote for a wi like experience. Who knows.
My games need me to place the iPad (regardless of size) on my lap or table, is that still ok/better? (I was not sure what you mean by being hard to hold; are you referring to handling the iPad like an iPhone/handheld game consoles?)