One specific issue with iPad multi-windowing is swiping up closes both splits in a space (and new windows really like being opened in split view). I think it would be better if the swipe up acted on a window (a split) instead of the space.

Or, you know, if there was a standard keyboard command to close windows. github.com/douglashill/KeyboardKit/blob/master/KeyboardKit/KeyboardWindowScene.swift

I would create a lot more new windows if I could close them conveniently.