I am also wondering about the “flat V” configuration. I know that a smooth engine will have a bank angle of 720° divided by the number of cylinders. So a smooth V-12 should have an angle of 60°. A smooth V-10 would be 72°. A smooth V-8 is 90°. A smooth V-6 would be 120° but that doesn’t fit under the hood of a car, so they usually make them 60° (smoothness compromise) or 90° to share manufacturing tooling with the V-8s. The 120° geometry can also be achieved for a six cylinder engine in the straight (inline) configuration.
The next step (the only “flat V” that I can think of) would be a four-cylinder engine. The best bank angle for a V-4 is 180°.