I like the 13.3" screen size (rather than 12.5"), just want it in a tight package, this is pretty close.
The bezels on the sides are narrow enough, I don't care if a tiny amount more could be shaved off or not, it's fine.
The bezels above and below are an area where some space should be reduced. The XPS 13 is smaller, but the X390 has the buttons for the trackpoint, and maybe a bigger touchpad (?), so it may not be possible to reduce the size as much. However, I think 5-10mm or so could still be shaved off without compromise. Not the end of the world, but nice to have, particularly with the other models in the range at similar sizes.
It would be nice to have the "standard" keyboard in place, as there is quite a bit of space either side. Again, not essential, but nice to have (the old IBM X-series models always looked great with edge-to-edge keyboards). I read someone comment somewhere that with laptops so thin, it's hard to have a keyboard extend to the edges, and have space for the sockets on each side - don't know if this is true or not. Frankly I wouldn't care if the laptop was a few mm thicker to accomodate this, leave ultra slim for the X1C.
Having a free RAM socket would be really useful though. I guess a single 32GB soldered DIMM is going to be expensive, so 16GB soldered + a free socket would be good (nothing soldered and two sockets would be even better, but that is heresy these days). This is probably the worst aspect of the X390, but it seems most laptops are going this way :-(
Only one 4K monitor is a bit odd.
The T490s / X390 / X1C lineup is very close on features and dimensions, though getting a decent amount of RAM in an X1C makes it rather expensive.