It would be great to have a keyboard shortcut to zoom to 50%. Often I want to see my artboard with some black/background around it instead of as full-screen or actual size.

I currently do this by Ctrl+0 to fit to screen and then Alt + scroll to zoom out a little. But it would be great to cut this down to a single step.

Ctrl + 5 is not currently defined in Illustrator default hotkeys, so it would not interfere with any existing commands.