Previous entries in this Io exploration series discussed overall mission constraints, science goals, and one proposed mission. It's likely given budget constraints and many interesting solar system targets, that we will get just one mission in the foreseeable future to explore any of Jupter's moons. If the Jupiter-Europa mission is chosen as the next Flagship mission (and it would include both closeup and long distance Io observations), then I would not expect to see either the Io or Ganymede observers chosen as future missions.
Let us suppose, however, that Titan is chosen as the destination for the next Flagship mission. That leaves Jupiter's moons as viable targets for future Discovery and New Frontiers missions.
Based on the ground rules of those missions, we would expect that one moon would be the focus (my vote is Io, but there are Ganymede champions, too). We have become used to extended missions that greatly expand on the goals and science of the original mission. Could one mission do the holy grail and explore multiple moons? In theory, the flybys could be done in an extended mission after the initial mission studying Io was completed.
Gravity assists would certainly enhance the possibility of missions that would visit multiple moons. If the craft plans to explore Io, however, and reduce radiation explore by flying a highly inclined orbit, then there is a catch. As I understand the rules of gravity assits, it would be hard to use Io flybys in an inclined orbit to set up subsequent encounters with the other major moons. (Although the mission design wizards might surprise me.) Instead, the craft might have to carry enough fuel to raise the perijove from Io to Europa or Ganymede. That's not impossible. Galileo raised its perijove by a similar amount after entering Jovian orbit. If the orbit was raised to encounter Ganymede, that moon is large enough that the inclination could be lowered to enable encounters with Callisto or Europa. Such a strategy, however, requires the craft to carry the extra fuel, which means a tradeoff of a larger launcher or less payload. It is doubtful that a proposer would suggest making that tradeoff to enable a possible extended mission that might or might not happen.
Another issue is instruments. The priority instruments for Io differ from those for the icy moons. For Io, for example, a thermal imager to map volcanic activity is a high priority. Its a much lower priority for the icy moons where it would be used to search for any hot spots. (Hot being relative on frigidly cold moons.) For the icy moons, near infrared spectrometers and ice penetrating radars are higher priority instruments. The infrared spectrometer is a mid-priority instrument for Io, and I haven't seen the radar listed as a priority for this world. The tension on instrument payloads extends to the details. Cameras typically carry filters to image in specific spectral wavelengths to image specific materials. This is not my field of expertise, but my understanding is that the filters desired for Io and the icy moons differ.
The Io Volcano Observer shows that a very tightly focused Jovian moon mission could be done on a Discovery budget (~$450M including launcher) if NASA relaxes budget constraints (in the case of IVO by supplying the power system at no cost to the proposer). If I read the slides describing the proposal correctly, the budget is very tight, with it unclear whether the imager, for example, would have all the filters that would optimize observations. Useful instruments such as a wide angle camera for stereo images or a near-infrared spectrometer are not on the proposed list and appear as wishes. If we are looking at a mission that would cover the science observations of multiple bodies, then the Discovery budget feels inadequate. We would probably need the resources of a New Frontiers ($650M not including launcher) to do an adequate job.
Europa has clearly been listed as the highest priority Jovian moon. If the next flagship mission goes to Titan, then a Europa observer that would do multiple flybys would logically be the next priority for a Jovian moon mission. The goals of the Europa mission would likely be similar to those of the proposed Ganymede observer. Much more of the moon could be imaged at high resolution than was possible with Galileo. A modern spectrometer (Galileo’s instrument was mid 1970s technology) could possibly determine whether or not ocean material lies near the surface. An ice penetrating radar could sample small transects of the surface during each flyby to test how thick the crust is.
If a Europa observer were to fly, extending the mission to observe Ganymede and Callisto would be easier than with an Io observer. The radiation field at Europa is less intense than at Io, possibly allowing the Jovian orbit to be in the plane of the moons. If so, then it would be easy to use gravity assists to arrange flybys of Ganymede and Callisto. The instruments tuned to study Europa would also be well suited to studying the icy surfaces of those other two moons.
Discovery and New Frontiers spacecraft are implemented under tight budget constraints. Proposing instruments for a possible extended mission would be a gutsy move in a highly competitive environment. I would love to see a proposed New Frontiers Galilean moon observer, which is what would be needed. I doubt that would happen. Instead, any mission proposals for Galilean moon missions are likely to be tightly focused on a single target. So if Titan is selected as the target of the next Flagship mission, then we would want two follow on missions to study the moons. One would be optimized for Io (and most importantly be placed in an orbit to minimize radiation and increase the number of flybys) and then other for the icy moons.