I have used a todo app for decades.
My todo list includes simple tasks, repeating reminders, projects with multiple todo items, etc.
I keep my todo list on my iPhone. I have no need or desire to share it with my other devices or with other people.
For several years I have been using TODO by Appigo. However, a year or two ago they FORCED me to "upgrade" to their cloud-based version. So now, all my todo's are in the cloud instead of on my private device.
So I started looking for a new app. These days however, every single todo app keeps your data in the cloud, including even the "reminders" app that comes with iPhone.
So if my data MUST be in the cloud, it would nice if I trusted the domain that hosted it.
So I am suggesting that you create a TODO app.

Now that ProtonMail has a Calendar, it would be good to have a To Do feature as well. It would be something simple, but extremely useful for productivity scenarios. Just a simple list of tasks with categories and "due dates".
The due dates could be real dates, or just today, tomorrow, next week, backlog.
The tasks could have categories as well, like kids, work, spouse, house, ... .

+1 for me as well - once less reason to use Google!

that would be great!

A massive +1 for me !!

A massive +1 for this.

yes indeedy

I would like to add some to-do lists as well to my secure mailbox.

To-DO lists are going to be a good add to the service.

I'm using ProtonMail already as Todo List. If a task comes in, I tag it with the label "Todo". This works for me, but it would be more elegant if I could enter a deadline, a reminder and the possibility to postpone the deadline to the next day if it's already reached.

Maybe this could be implemented with a special group of labels, which add this functionality to every mail which is tagged, until this label will be removed (then it could be a normal e-mail again and will be removed from the tasklist).