I don't think that printer manufacturers are going to add a connector for people to swap in Discov3ry, unless you do deals with them so that it's in their interest to do so.
But perhaps you can introduce the connector so that it's easy to add/remove the Discov3ry. I'd love to be able to use my printer for FDM printing, then plug in the Discov3ry to print specific things, then back. So if you guys came up with a module that I could wire into my current printer controller, allowing me to plug in the Discov3ry and have it take over, then unplug it and have the standard extruder take over, that would make it much easier than swapping wiring to the controller board each time.
As for what else might plug in, one thing I can think of is a laser. There are some kits for mounting a laser into a 3D printer as a low-powered cutter, or for resin printing.
Another possibility is something like a dremel, using the 3D printer for X/Y/Z motion, and using the port to power the drill. Of course, only for low powered applications, since the printers aren't designed to move the mass of a real CNC milling head.
Potentially swapping in different extruders entirely, such as between different nozzle sizes or filament sizes?