I'm pretty sure SATA Express will not come to the Mac, not Apple's game.
Don't think 2 SSDs will be supported as well, higher capacity drives maybe, up to 2GB possibly.
RAID would be nice for some, but won't be coming I say.
Too few lanes to make it happen, at least in a decent way.
We'll get Haswell starting with 16GB instead of 12GB (come on Apple, why 12GB in the first place?!), same number of cores to choose from, although Haswell can go as high as 18 cores. I don't think Apple will go that far, even as BTO.
An 8 core would be fine for me, even overkill. Not sure if I should max out the RAM already, or just stick to 32GB, don't feel like upgrading later although 32GB should suffice for the lifetime of the machine I guess.
AMD Fury based cards would be sweet, clocked down the should fit perfectly within the power envelope. The current thermal dissipation system should be more than enough to keep it cool and without throttling. Something Nano-like...
I also believe NVidia GPUs won't be coming to Apple anytime soon now. You can always use their own drivers.
NVidia does beat AMD in a lot of metrics, the option at least would be nice, but forget about it for now.
The best option right now for the Pro guys would in fact be based on Grenada still, although Fury would be nicer although people forget this is not a gaming machine
Yes, TB3 unfortunately is not DP1.3 compliant as far as I know. Too bad. I do believe you can do some magic with both streams of DP1.2 to get 5K@60 on "a single cable" though. Or you can have 2 4K@60.