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).