It wouldn't be that major of a project with today's equipment. Just attach any modern camera. However, most of those missions were designed around equipment available decades before they are actually being used, so they are outdated by today's standards but were high end back then. Also, B&W sensors with filters have so much more resolution than dividing by the RGBG pixel arrangement. Maybe Carl Sagan will have had enough of an impact that future mission will include a "regular" camera specifically for the function of making everything more human relatable.
There's also a limit imposed by the ability to return data to earth. I'm thinking of New Horizons, for example. The spacecraft took about 8000 measurements or photos in it's rendezvous with the Pluto system. It totalled about 6 GB of data that took 15 months to return to earth (with a maximum effective rate of 2kbps). Obviously Saturn is closer, and you could get a better transfer rate, but at some point you hit a limit of how many pictures you can take versus what image quality you can accept.
I'm willing to take those limitations. While Pluto is just Goofy, we could start with the other planets. What would Jupiter's clouds look like? Venus? We receive much more information from the orbiters of these planets, so hopefully on future missions (please let us keep them going) we can get these images. Honestly, I'd be happy to receive a JPEG from a modern DSLR just to save space, but obviously the RAW would be ideal. After all, if we're doing this, let's do it.
