For sure coin mechs - offer an upgrade kit for Ops that need/want it.
Side art (cabinet and head) - make it optional - a set of decals that can be purchased separately
Topper - optional
Shaker - optional
All of this (and more) could be check boxes at time of order. But this could introduce more complexity and forethought on the manufacturing side unless you leave it all for installation by the customer. I'd be okay with that. You could also use this as a revenue stream for the distros to install this stuff prior to shipment...