If during the planning and construction of the Voyager missions they knew what we know now, what changes to the missions would have been most helpful in augmenting the science obtained?
Knowing what we know now about Titan's thick and resistant-to-surface-photography atmosphere, choosing a Pluto flyby for Voyager 1 over a Titan flyby would perhaps have given more exciting results. Voyager 1 had the capability of flying by either but not both of these, as going by Titan gave the wrong exit angle from the Saturn system to subsequently fly by Pluto.
Instead, we had to wait until 2015 to get a closer look at Kuiper belt objects with new Horizons.
A simpler question is what wouldn't have changed. The answer to that is simple: The power source, and possibly propulsion. We still use pretty much the same radioisotope thermoelectric generators (invented in 1954) that powered the Voyager spacecraft to this day. Whether the Voyager spacecraft would have used the same kind of propulsion that it did use is perhaps debatable. I suspect they would not have used the propulsion systems used on Voyager if they had a choice of modern alternatives.
Cameras have improved a lot. If the designers of the Voyager spacecraft had the choice of using the camera technology of today versus what they had then it would be an easy choice. They would have chosen the camera technology of today.
The biggest improvement since the Voyager era is in computation and data storage. Computing systems, even the lousy ones that deep space missions tend to use / are forced to use, have changed by multiple orders of magnitude since the Voyager era. For example, the computers on the Voyager spacecraft had 32K words of memory. I'm amazed they did so much with so little memory.
The New Horizons spacecraft, which had a considerably lower budget than did the Voyagers, took advantage of those huge improvements in computing and data recording technologies. Computation and data recording were extremely limited in the Voyager era. Those wonderful images of Pluto from New Horizons simply would not have been possible using Voyager era technology. The New Horizons spacecraft took its sweet time (well over a year) to transmit those recorded wonderful images back to Earth. The Voyager spacecraft didn't have that ability.
I think the simplest answer with no "back to the future" aspects would be that what we know now is that the Voyagers are still alive an kicking and providing new science!
Thus they might have considered finding a way to include the mass and heat of a second RTG somehow to double their available electrical power and warmth in the 2020's and 2030s.
This is not at all trivial task and I'm not saying they'd be able to do this; those RTGs were heavy and of course produced a lot of heat, so it would be a significant design challenge for the spacecraft and the payload considerations during launch.
They might also look into adding more of the small radioisotope heaters for some instruments.
In the end they might instead say "nah, if this works there'll be plenty of funding for a second round of Voyagers in a decade, let's just get these out there for now.
- Which wears out faster on RTGs; the R's or the TG's? (the radioisotopes or the thermocouples)
- Why did Voyager have to shunt unused electrical power and radiate as heat?
- How cold are the Voyagers now? Colder than LOX? Colder than SOX?
- Did any of Voyagers' receivers' front ends take advantage of the "cold of space" to lower noise? (communications or radio/plasma science receivers)
- How long will communication with the Voyager probes continue?
- How would the Voyagers be tracked after the RTGS runs out of fuel?
- Is Voyager 2 capable of proving the existence of Oort cloud?