Sometimes you are adding technical debt to your project, a good way to keep track of these would be to mark them so you could easily track them down later.
A lot of programming languages use comments like "// TODO" for this. This could work inside annotation in Mendix maybe.
An even nicer way would be if you could add tasks to activities. Imagine right clicking an activity and have a "Add task" option. These could then easily be shown in a "Tasks" window similar like the existing "Find results" window.
4 years later some other great mind thought alike: https://forum.mendix.com/link/ideas/3202