I'm actually surprised Apple hasn't released a new retina Thunderbolt Display yet. They're already making the panels for the 27" iMac.
Not going to happen until there are Macs supporting either USB-C with DisplayPort
1.3 or Thunderbolt 3. Otherwise you'll need two cables. I'd guess that they'll stick with Thunderbolt, but who knows...?
I wonder if Apple will produce a Thunderbolt Display using Thunderbolt 3, or a USB-C 3.1 Gen 2 display (obviously they'll come up with a better name, or resurrect the Cinema Display name). If they do the latter, it will work with the existing MacBook, as well as any future Mac they produce with either USB-C or Thunderbolt 3, while still improving throughput over USB 3.1 Gen 1/USB 3.0.
OK - here's the complication as I understand it:
DisplayPort 1.2 needs two cables to drive a 5k display.
USB-C supports (at least in theory) DisplayPort 1.3, which can drive 5k over a single cable.
Pro: its electrically compatible with DisplayPort - the high-speed data wires in the connector switch to DisplayPort mode
Con: At 5k, only the legacy USB 2 pins are left for data, so you won't want to chain high-bandwidth stuff off the display.
Con: The GPU needs to support DisplayPort 1.3 (not sure if any of the current Intel GPUs offer this?)
Con: the rMB certainly can't support 5k this way.
Thunderbolt 3 only supports DisplayPort 1.2 - but it can send two DP 1.2 cables-worth of data down a single TB cable and support 5k that way.
Pro: Don't need a DP1.3 GPU
Pro: (I suspect) there's a bit more bandwidth left for daisy-chained devices
Con: In this mode, the signal isn't electrically compatible with DisplayPort: its Thunderbolt, and the display has to be a thunderbolt display, with a TB3 controller chip to split off the displayport data.
Now, I'm
assuming that the existing 'legacy mode' of Thunderbolt, whereby you can plug a DisplayPort device into a TB 1 or 2 socket and it morphs into a DisplayPort will be subsumed by USB-C's similar, but more flexible, trick whereby some of the physical data lines switch to DisplayPort mode. However, TB3 has only talked about DP 1.2 support - if the TB3 chipset can't deal with DP1.3
at all - even in 'legacy' mode - then you've got a messy situation whereby, when it comes to 5k support, TB3 isn't quite a
proper USB-C port and can only use Thunderbolt 5k displays as opposed to the (doubtless) forthcoming 5k USB-C displays.