Something that comes to mind when determining requirements. Are they really requirements or nice-to-haves? Be honest, this is no time for personal bias.

In the case of an aviation PFD, the first (#1) requirement is that the PFD does not interfere with egress from the aircraft. Only after #1 has been satisfied do any of the other requirements matter, even buoyancy is second to egress.

This is the reason I went to a purpose built aviation survival vest. The folks that make these vests for a living have been through this design process and in general make a pretty good product.