What would be everyone's next targets for probes to visit? For me:
Io (because I just find it fascinating)
Is that you and Rosaly Lopes?
She is a Planetary Science section manager at JPL, an excellent person. She has done extensive research on Io, named volcanoes there after Brazilian goddesses (she's from there originally), and you have to hear her speak about Io - such passion and enthusiasm is rarely seen.
But I'm afraid that Io has no serious science backing that can be used as a mission foundation
About the rest, I agree in general with Red Berret's take:
I definitely think we should send an orbiter to Neptune. As I mentioned earlier though, the real challenge is longevity.
If you look at Cassini, it's been functioning for 18 years, but 7 of those were just to get to Saturn. It has, at most, 18 months left (I believe due to propellant exhaustion). This is frustrating for me because the probe is fully functional and has plenty of power. If you look at this wiki article, transfer orbits back to Jupiter, or onto Uranus or Neptune are possible, although not likely options.
To explore Neptune adequately, you have to consider a minimum 40 year lifespan for the probe - most likely longer. Like I said, changes out there happen very slowly; you'd need your probe to observe for at least 20 years, and that's without cruise time. And once there they need fuel as much as they need power.
These are the natural challenges planetary exploration faces: developing sufficiently robust, long lived, efficient manoeuvring systems. Although Neptune having only one major satellite makes for a simpler task of putting the probe in a stable orbit if threatened with fuel exhaustion.
... but with some caveats.
ESA and NASA differ very much in their view for the future. NASA has their "Flagship mission" class, ESA has Class A (IIRC, was that the term?), which are the most expensive missions. A few years ago, I took (a bit remote) part in trying to formulate a joint project that can combine the resources of both institutions. It eventually came to nothing. ESA waned to explore Titan and Enceladus, NASA wanted Europa and Ganymede. ESA wanted to follow the hydrocarbons to the origins of organic life, which made a lot of sense, especially after the Huygens probe. Their proposal was also beautiful - an orbiter, a lander and a gondolfier (sp?; a flying balloon with atmospheric probes). But NASA wanted to take advantage of their huge resources already spent in the last 20-30 years in the development of radiation hard electronics. By radiation standards, Europa and Ganymede are far more challenging while Titan and Enceladus are considered a child's play (forgive the exaggeration). The picture below illustrates where Europa and Ganymede are in the radiation belts.
Pros and cons on both sides. A month or so ago, NASA made the announcement about Europa (previously existing as Europa Clipper project). Testing for technology development purposes have been ongoing for ages, and intensified before the announcement. All well, but to me, I'm not sure that the science return will be maximized. It comes to what Red Beret said - longevity. You all hear about missions like MER (Spirit and Opportunity rovers) that we designed to operate for 3 months and lasted 12 years. Sure but for one of these we have 10 of the opposite examples. The original mission for Curiosity was to drive 20km in the first year; we are now just past 10 km in the second year. Most science targets were met half-assed, but victory was declared. I hope the next rover is not such a piece of junk, to be honest; this is so much below our own standards, we can do a lot better with the reliability. SMAP that launched just a few months ago may be cancelled soon... Anyway, the point is that we cannot deduce longevity from one successful mission, we have to show a culture capable of building successfully long missions. IMHO, we as as far from that as we are from landing man on Mars.
One thing that Red Beret didn't touch on is that the long missions require long mission operation budgets, which eat up the funds from other missions (I had a post on that a few pages back. This is a serious problem. What organization will devote funds (at least $150M/year) 40 years into the future? That's $6bn. The worst case scenario is to build a spacecraft for, say Neptune, fly out for 10 years, the market crashes and NASA/ESA have budget cuts, go there, snap a few pictures and say "OK, we made it, end of mission".
