Quoted from pinballplusMN:When you consider the footprint of a playfield 43 by 21 and the amount of time that space has been utilized 60+ years its hard to come up with new stuff to fill that space.
Like OP said, Keith really did a great job on his two games. Very fresh in today's time. Also, having played a few home brews, there are still a lot of creative things that can be done. It's more of the "risk" factor with making a playfield design that is too unconventional. Stern knows that there is a certain formula that shoots well and feels good. They're the pop genre of pinball. They know that something works, and it makes them money, so they rinse and repeat, like what they play on Top 40 radio.