What is a “must have” depends on your use case, personal preferences, and the shortcomings of your distro’s default configuration (I’ve never used Cachy so I don’t know what’s missing).
For myself, I usually end up installing VLC and Strawberry Media Player, since the media players most distros come with aren’t as good. On non-GNOME distros I tend to install GNOME Disks as it’s the least painful to use of the GUI partitioning tools I have used. My preferred rich text format is Markdown, for which I use ghostwriter. I also usually install a few FOSS games to pass the time with - my favorites are Freedoom, SuperTux, SuperTuxKart, and Xonotic - and RetroArch for emulation.