The current thickness of the MacBook lid (my 16") is <3.5mm, and the faceID module is >3.5mm. Given that you need glass and metal in that 3.5mm as well as the faceID module, you're talking about significantly changing the dimensions of the MacBooks to do this with today's faceID module.

I look forward to the day they do get it fit in there though (hoping that the Silicon redesign allows for it)!