Remember the Thunderbolt Display? Because sometimes it seems like Apple doesn't; it's been nearly five years since the last time the monitor-plus-laptop-dock was updated, and it's increasingly an anachronism as high-resolution Retina displays take over the lineup. For some time now, the Thunderbolt Display has actually been larger and heavier than the 27-inch iMac, which has a whole computer inside of it.
One of the roadblocks to releasing a new 5K Retina-capable version of the Thunderbolt Display is the DisplayPort graphics interface, which in its current iteration (1.2) is only capable of driving a 4K display at 60Hz over a single cable. Even if Macs picked up Thunderbolt 3 ports, they still wouldn't support DisplayPort 1.3. Apple had to develop its own timing controller to make the built-in display work on the 5K iMac, but it can't drive an external display of the same resolution, nor can any current Apple laptop.
One potential solution, according to a report from 9to5Mac, is to put a dedicated GPU in the display itself—this would allow Apple to use the same timing controller from the 5K iMac to drive a 5K Thunderbolt display. This might have seemed like a strange solution a year or two ago, but Thunderbolt 3's adoption in the wider PC industry is already making it possible to use external graphics enclosures like the Razer Core to add dedicated graphics to thin-and-light Ultrabooks and mini PCs. Changes coming to OS X will allegedly allow hot plugging of one of these displays, switching seamlessly from the integrated GPU in the laptop to the dedicated GPU inside the monitor. Whether that dedicated GPU could simultaneously drive the Thunderbolt Display and the laptop's built-in panel isn't clear, but it should at least be technically possible since none of Apple's current laptops have 4K-or-greater displays.