The classic place for the license plate is the center.

And since the license plate dimensions are given, there is little room for design freedom here. So it's best to place it in the immediate vicinity of the taillight, below or above it, depending on where the window for the license plate illumination on the taillight is located.

What was Harley-Davidson thinking with the LiveWire's license plate holder?

Honestly, we don't know. It was probably a mixture of "The Germans and Italians do it that way, too." and "Crap! Now the seat is hinged, but the engineer didn't think of the license plate holder ..." Some think it seems to have been sheer desperation. But we wouldn't be W&W if we didn't have a well thought out and appealing solution that also offered shapely bullet turn signals: the SpeedFire Lightning II Kit.

What do I have to consider for license plate holders?

The license plate must (Germany, Austria, Switzerland equally) be fully legible, even at an angle of 30° from the side, and it may only be tilted a maximum of 30° to the front and 15° to the rear. For the sake of completeness: its lower edge must be at least 20 cm above the road and the upper edge max. 150 cm above the road. (Directive 2009/62/EC)

But there are no rules on material, shape, etc.: whether between the cases, on one of the cases, on the fender, an extra holder ... does not matter. The main thing is solid! If you lose the license plate during the ride, you may also be fined in addition to all the hassle with the authorities to get a new plate.