This looks absolutely incredible, Eventus! Congratulations on making it this far to beta with your mod! My four ideas for Kerbodyne+ parts are an expandable, non-inflatable centrifuge for titanic stations and interplanetary craft, 3.75m NERVAs in addition to the giant ion engines, and an absolutely MASSIVE fusion drive for interplanetary travel. For the centrifuge, it should start out initially compact and 12m in diameter, expanding out to over a hundred feet in diameter, capable of housing 20 kerbals, 10 for each of the two sections of the centrifuge. I imagine the centrifuge to look like the centrifuge from the space station from the trailer for the Infernal Robotics Rework, "Sampling Duna," but Kerbodyne-esque, more streamined and with toggle-able lights: https://i.ytimg.com/vi/yxh9En_Rd6E/mqdefault.jpg As for the fusion drive, I'll let you decide on the design.