Can I use 2019 iMac as monitor for Wii/Wii U? Does anyone know of a setup to somehow plug my Wii into my 27' 2019 iMac? I'm fine with any strategies here, no matter how complex.
You could use a USB video capture device to capture the video output from your Wii or Wii U. 
A device like this with support for macOS will show up in QuickTime, which you can switch into Full Screen, and could allow you to use your 27" iMac as a display.