> calendar you can add events to (does a modern desktop calendar for Linux even exist)?
I don't know about Gnome - but KDE has both a desktop calendar/organizer app (Korganizer) and a (unrelated) Plasma widget for the toolbar - the gorgeous Event Calendar Plasma widget, which shows your Google Calendar events.
So basically, the same situation as with mail clients, photo apps, music players, ... I wish the GNOME world would decide to just merge all the features into one app per task and then kill the others.
Email Client: None, users still using desktop clients know what they want and how to get it.
Terminal: No preference
IDE: None, this should be chosen by the user if they want one.
File manager: No preference
Basic Text Editor: No preference
IRC/Messaging Client: None, same situation as email
PDF Reader: No preference
Office Suite: LibreOffice
Calendar: Like the calendar the clock opens (no preference) or a calendar you can add events to (does a modern desktop calendar for Linux even exist)?
Video Player: VLC
Music Player: Whatever is least bloated
Photo Viewer: Whatever is least bloated
Screen recording: None, most people don't need or want this.