The derailleur plug-in already exists, but they're so hush-hush about it. Trek and Scott had them on some e-bikes. They apparently have a tiny battery to give you 100 or so shifts in the worst of situations. Dropper is going to require quite a redesign and space is already at a premium at the bottom of a dropper. From what I was told, SRAM went with the Shimano motor mounting pattern for immediate and easy installation onto existing bikes. Makes a lot of sense to flood the market quickly, especially if there's no real issue with the mounting pattern. Ruh-roh...