They're keeping a really tight lid on the "Surface Pro 4". There's literally not one single shred of leaked info on this device anywhere on the Internet. A guy that I know who works for MS says the new Surface is much more than anyone is expecting. I'm expecting a few more surprises as well.
I don't know how much I believe that right now. We've already seen mentions of 12" and 14" iterations. Maybe a Surface Mini is the surprise kicker. Beyond that, I don't expect that much would be different. You probably get Skylake, an improved Type Cover (PLEASE, I hate the current ones), and maybe the kickstand gets some added build quality, I don't know. Windows Hello will probably get an iris scanner into the thing like the phones (since they can't fit one of those Intel cameras in it, I'm guessing).
I just stop and ask myself what would be a pleasant surprise with the SP4 that might not be expected, and I have nothing. A better keyboard is all I think it needs, and I don't care a tiny bit about Windows Hello. I guess the only major surprise would be if they got PureView optics and OIS into the rear camera, maybe with dual flash or something.
Otherwise, like I said, I basically expect:
--12" and 14" offerings, maybe 2K on the former and 4K on the latter (or 1080/2K)
--Skylake chips
--Better typing solution
--Iris scanner (I have a hard time believing they'd put it in the phones and not the lead W10 hybrid device)
--Maybe dual USB ports (possibly Type-C for one)
--Possible Surface 3 Mini (or a dual-launch of Surface 4 and Surface 4 Mini, if Skylake Atom stuff is close to completion)
I'm honestly at a loss for what could be surprising.