Gigabit ethernet. Video converters for each external monitor type, plus for the projector that you only use twice a year. The external mouse that you really like. The phone. The 2TB USB3 hard disk that you already have. The external DAC and pre-amp that turned your Mac into a high-quality 4 channel recorder. The non-bluetooth wireless mouse that you like so much better than Apple's. The SD card for the video camera that your kids use.
Forgot your phone/laptop charger? You can borrow mine as it will be the same USB-C. Yes, now it's painful, but in ~10 years it will be glorious. However, the problems of the switch are happening now while the benefits will come unannounced, which is a pity.
Currently for me, your list of examples is already three dongles (minidp-hdmi for hijacking hotel TVs when I'm travelling, minidp-dvi for my monitor at the office, minidp-vga for the projectors in the meetings rooms), and five different USB cables - usb-B for the usb hub in my monitor, lightning for my phone, micro-usb for my SD card, mini-usb for my camera, and micro-usb3 for my harddrive).
Everyone's annoyed that they're replacing 'The One Plug' with usb-c. But 'The One Plug' is a myth. For me, the only difference between a different connector for each device, and a different dongle for each device, is what spin you're putting on it. Either way I'm still going to need 8 cables in the bottom of my bag. The donglepocalypse is just the same list s/usb/usb-c/.