At first I thought this may be out of scope for Proton, and perhaps it is. But I have searched for years for something like Todoist but open source and end to end encrypted. There are many apps in this space, but I mention Todoist because it has the ability to function as a project management app (for a freelancer or small team) while still being usable for day to day tasks. Take that functionality, give it Proton privacy, and integrate it with Drive, Calendar, Mail, Standard Notes, etc, and it would be a powerhouse.

To be honest: I will not continue to use proton (having tested it for a few weeks now) only because of this. I do not understand, how you can concentrate expanding your product suite with VPN and Pass (next to many good alternatives) while not creating a todo-app and perfecting the mail-calendar-todo-combination on all devices.

I would only suggest taking Google Tasks as a model with the "all-lists-on-one-board"-style. There should be labels too (in addition to categories = lists). And there should be a function to be reminded on a specific date.

If I get an E-Mail, I should be able to make a task or a calendar entry with one click (maybe a few more but only in cases I do not what e.g. the standard-todo-category set by myself prior).