I took a quick look at display-line-numbers-mode (nice work as always) and had a few thoughts:
1. The similar linum-mode when possible displays a subtle vertical separator between the line numbers and the left margin of the text area. This helps to visually parse the numbers as separate from the text and helps identify where the first text character is. It would be good to have something similar at least as an option.
Then linum-mode should be removed (maybe add an alias) so there is no confusion as to which mode should be used.
2. The mode-line popup menu that displays/hides line and col numbers in the mode=line could be expanded to toggle this mode on and off locally and globally, as well as providing options for its various display settings: relative, visual and absolute.