I don't know if it doable with the current architecture, but it would be nice to have an improvement in the way multi-screen is handled.
It is already very good, with the exception of windows that can randomly appear or move to any of the added screen instead of staying on the primary screen.
This can lead to embarrassing situations during a public presentation, for instance.
Gnome, as far as I know, is the only DE that handles it well.
Thanks for any feedback, positive or negative.