I have to add, a 5K display would kill off the whole designer market, which I believe is one of the core target groups of the XPS series. It is a direct MacBook Pro competitor after all, blatantly so, down to its design.
The reason is that Adobe Creative Cloud apps do not support UI pixel scaling higher than 200%, and using a 15" 5K display would require to squint to extremely tiny pixels. Even the 4K at 200% is somewhat pushing the limits, but is still very usable.
Bottom line: there are basically just no benefits at all for a laptop to sport a 5K display right now.
120 Hz, no. Not going to happen. Dell has AlienWare brand for those. Besides gaming, I can't see any real benefit, and in gaming the laptop can't cope with the performance requirements of the 2560 @ 120 Hz, which is basically the same as running 4K @ 60 Hz, which is far beyond the capabilities of all but the bulkiest gaming laptops (think GTX 1080 Ti).
But a G-Sync pane , please.