The possibility to insert annotations everywhere. This could be implemented by adding a textarea for comments in the Inspector Panel, since all objects in Bitwig show related information in the Inspector Panel when they are selected. Optionally, a icon could be displayed on the objects with annotations.
Annotations are often necessary, especially for complex projects or projects meant to be shared with others. The Grid has a Label and a Comment modules, but these features are missing elsewhere in Bitwig. People have to either take annotations and store them elsewhere, or rely on their brain and forget the details with time.
Very well, as proven by the fact that The Grid comes equipped with this feature. The need for documentation is just the same outside of The Grid.
Outside of The Grid, the only feature remotely similar are cue markers, which are clearly insufficient.
MNotePad by MeldaProduction, which is free. The problem is that you need to press shift+spacebar if you want to enter a space because spacebar by itself starts/stops playback. So a built-in option would be definitely preferable.
GitHub - OpenMusicKontrollers/notes.lv2, but Bitwig does not support LV2 and the project seems to be discontinued.