Given limitations of 'Unity asset flip made in 10-20 hours; game does not have to be polished or particularly good, merely functional', what would be the easiest genre of 3D game to make?
Probably some sort of first-person walking sim. The first person perspective lets you avoid the need for any assets for the player character, and the walking sim format is amenable to simply turning off physics and collision detection entirely and letting the player move the camera around directly on a 2D plane, with some basic bounds checking to prevent them from moving it inside of objects or terrain. Throw in some NPCs represented as shitty flat bitmaps superimposed over the 3D landscape (it's a valid stylistic choice!), and use an off-the-shelf dialogue plugin for all interactions – no need for animations when you can just have a text box pop up and describe what happens in florid, typo-ridden prose. (The typos are also a stylistic choice.)