Accept calendar invitations from others. I use the webbased ProtonCalendar for some weeks actively now. But I find it difficult to switch to this calendar app. Besides reasons stated in other suggestions here (no mobile phone app or integration with a mobile calendar app and no notifications), I would like to have the option to accept calendar requests sent by others to my ProtonMail. I have tested this today by sending myself a calendar invitation from my gmail and my office365 and my work Exchange accounts. All 3 invitations ended in my ProtonMail inbox as ics download files without the option to click "accept" and have it in my ProtonCalendar.
In order to make ProtonCalendar to work with ProtonMail it would be great to see the option to add calendar invitations to ProtonCalendar right out of the ProtonMail inbox by clicking "accept".