I look forward to one day buying a rMBP 15" with Iris Pro graphics and a thunderbolt dock from nvidia with TB3 and their latest midrange graphics card built-in. Now, they have all the pieces in place to do this.
Apple would more likely put a high-end GPU into their 5k monitor and sell it as an upgrade. That way, they don't have to worry about TB3 being limited to DP1.2, as they just have to transfer GPU data back and forth instead. I just hope they include a way to upgrade the GPU inside.