What is expected:
-> Better camera with auto-focus feature. Why? The Surface non-Pro 3 has one, if I am not mistaken.
-> Skylake CPU. only ~5-10% performance boost, but the GPU is expected to be more powerful (sadly, crappy drivers, remains crappy). Potentially reduce system heat, as the GPU should have easier time rendering things, and when not in used, the CPU is more power efficient. This should also increase in battery life (probably 30min at most, assuming identical batteries used from the SP3).
-> More RAM in the available options, and DDR4.
-> Improved pen tracking (Surface 3 has that)
If rumors are to be believed with my speculations:
-> Surface Pro 4 will have 2 sub model: Core M for the lowest end mode, replacing the Core i3 option, which will feature a fanless design, and be a bit thinner, and Core i5/i7 like now, with fan, and thicker (due to heatsink and fan. probably be the same thickness as Surface Pro 3). Alternatively: nothing will change, and the Core M rumor one, is a Surface Mini resurrected with Intel CPU.
-> USB Type-C 3.0 or 3.1. (probably USB 3.0, as Intel doesn't support 3.1, and putting a custom chip would cost space on the device)
My wish:
-> Custom Nvidia GPU, using the GPU of the Nvidia Tegra X1, which consumes only ~8-10W. This won't be this massive powerful GPU of course, but should be more powerful than Intel best offering, have proper and full DirectX, OpenGL, OpenCL, DirectCompute, PhysX and CUDA support, and most importantly, significantly better drivers. As for cooling. 8-10W chip doesn't need a fan solution to cool it off. It just needs to spread the heat it produces. It can also have switchable graphics to not affect battery life when not gaming or using any software that demand a more powerful GPU solution. Of course, I expect this Nvidia solution to only be with the high end mode of the Surface Pro 4, like the i7 chip.