In addition to the upper playfield module, both the side habitrails and side target assemblies are also modular and can be easily changed. There is nothing preventing you from, for example, utilizing cross ramps where shots return to the opposite flipper. The flipper and slingshot mechanism is also modular to the extent that it is easily removed and replaced, so with the proper engineering you could do a Harlem Globetrotters-type of 3 flipper design if you wished. Other than the limitation that things must float above the playfield to allow the IR ball-tracking to work, the sky is pretty much the limit. I'm working on some initial design ideas for an upper playfield, ala Black Knight, for a game of my own design.
That's the other huge plus for me - user programmability.