DisplayPort/Thunderbolt output for high-res external display
The XPS13 already has DisplayPort, which is great because it not only works with high-res (2560x1600, 2560x1440) monitors, but also allows connection to HDMI, DVI, and VGA with cheap, passive, adapters.
The only thing better would be Thunderbolt, so a single cable could be used to connect to not only a monitor, but also a dock with gigabit ethernet and multiple USB ports (like the Belkin thunderbolt dock, or like a apple thunderbolt display with all the ports built in), and potentially to daisy-chained monitors.
If you can get all the necessary drivers for Thunderbolt working and pushed upstream into the kernel, Thunderbolt would be ideal! If not, *please* at least keep DisplayPort, *do not* put HDMI/VGA/DVI ports on the laptop, they don't work with high-res monitors, and the adapters to convert from DisplayPort to HDMI/VGA/DVI are less than $10.