The spacecraft itself isn't a particularly big deal on these missions. They will usually use an existing bus or put together existing subsystems. So basically t

The spacecraft itself isn't a particularly big deal on these missions. They will usually use an existing bus or put together existing subsystems. So basically they already do as you suggest. Cassini, currently around Saturn, is directly descended from the Mariner/Voyager/Viking line, as were Galileo and Magellan. MAVEN, MRO, and Mars Odyssey are all basically the same type. Mars Observer was a comsat, and most Earth observers use a commercial sat bus. Historically the Explorer, Mariner/Voyager/Viking, Pioneer, Ranger, Venera, Zond, and other families shared quite a lot. Most of the effort goes into novel and usually hand-made sensors to deal with specific scientific questions. There's not particularly a point in sending the same instruments to the same places again, and the accessible parts of the solar system are fairly well studied from a generic standpoint. If you're going through the (fairly expensive) effort at all, you want better or different instruments, and that's what takes so long. There's a reason all these programs are led by a "principal investigator" and not a "head engineer". I do think a common design with fairly generic instruments would be useful for a pair of ice-giants orbiters. Uranus and Neptune are basically unstudied up close, compared to all the other planets. But two does not mass production make.