The problem is even if they do go with Thunderbolt 3 (presumably with usb-C connectors) they can't support Displayport 1.3 or 1.4 yet. Unless they create their own chips or something, which I guess is possible but doesn't seem likely. It seems strange that Intel is taking so long to support Displayport 1.3.
Even the New Mac Pro loaded with dual Polaris GPU (dp1.4 capable) wont have DP1.3 even less DP1.4, NEITHER IS NECESSARY, a 5K Display can perfectly be feeded by 2 DP1.2 Signals (aka MST) this don't impede having 60fps refresh neither gaming (just the GPU has to be capable to split game display to "two screen", pretty common Metal also is fully capable to do that), What does SST (feeding the entire 5K panel with a single signal) is to save Money and an extra cable, at the potential expense of more line noise.
Don't care about D.P. 1.3/1.4 it's more GPU Spec-War gimmick than actual practical need.
On Dual D.P. 1.2 (MST) you stil can drive a 5K Panel with 10Bit color space (aka HDR) w/ANY-Sync at 60 fps, this is how the MP6,1 does (very well) futher on TB3 you'll only need a single cable (TB3 has dual DP1.2 channels).
Further, did you know NONE Integrated GPU from Intel even next Gen Kabby-Lake is DP1.3 capable, even cant support more than 4K on DP 1.2 SST (but on MST Iris Pro are capable to drive a 5K/10bpp Display at 60p plus a 4K/10bpp Display at 30p or at 60p/8bpp at same time).
Further Macs with an TB3 display no only drive the 5K Panel at 60fps, also feed it witn an usb3 channel and a TB1 channel you can daisychain to other devices.
Even it's rumored some future macbooks pro and iMac to support an new advanced Target Mode, sharing both storage, display and input device with they clients, as emergency display/input for mac pro, mac mini or other iMacs, and those would be capable to keep it charged powering it thru USB-C's Power Delivery.