I wonder if it would be practical at some time to send a probe to survey some of the 55+ outer moons of the Jovian system? Although these are fairly widely spaced, I imagine that they're in a more compact area than the asteroids of the Main Belt, and that an orbit could be devised to intercept a half-dozen or so of them. This wouldn't be a very exciting mission, as most of the outer satellites are doubtless small rocky/icy irregular bodies, but I can imagine it standing in for a survey of the asteroid belt (since many of these satellites are probably captured asteroids) and just possibly they might show more variety in composition and structure than we might expect.
I'm not any good at orbital mechanics or fuel consumption calculations, so I wonder what the practical difficulties of doing such a survey would be, and what the costs might be in comparison with an inner-system survey like Galileo.

This sounds like a pretty interesting idea to me. Probably something to launch ten to twenty years from now, using a solar powered ion or plasma drive, and carrying lots of tiny landers to do some sample analysis. The mission might take twenty years, but could probably orbit most of these moons, and do a fly-by of the ones orbiting in the opposite direction.

I'm not sure what we'd get from a complete survey of the outer moons, except perhaps some more detailed clues about the origin of the planets, and perhaps a list of the moons, with unusual chemistries.

IIRC, there are spectrographic studies showing similarities between most of the outer Jovian 'rocks' and the repective Trojan clusters and some asteroids in the outer belt.

And surprisingly, Iapetus.

{I for one would not be surprised if the dark crater bottoms of Hyperion show a correlation spectroscopically too}

Any outer excursions of Cassini come close to any outer Saturnian 'rocks' ?

Since the captured moons of the outer planets are captured asteroids and/or KBOs, I strongly doubt you'd learn anything from flybys of them that wouldn't be learned more easily by flying by such objects wherever you find them in the Solar System. (There IS, however, enough interest in studying the specific composition of Jovian Trojan asteroids and Centaur objects that the Decadal Survey recommended a future New Frontiers mission to fly by one example of each -- a goal which might be achieved more cheaply instead by having other outer Solar System craft make opportunistic flybys of such objects during their missions.)