Prompt for follow up task
Very often completing a task will raise another one, but it can be very easy to forget to input the follow up - especially if you are the type of person that needs todoist!
A simple but boring example: task "Agree with customer what tech demo(s) to present at trade show.", follow up "Write up plan to implement and install tech demo."
I think a really simple addition to todoist could be very valuable in this scenario: when you tick the completed box todoist should prompt you "do you need to input a follow up task?" and have an 'add task' box ready and open at the same level in the same project.