A variety of interplanetary probes can be supported within the backbone system. These are placed into interplanetary Hohmann transfer (or gravity assisted) orbit using solar ion propulsion (the ion engines might be recovered). They use aerocapture at the target planet, except at Mercury and Pluto. This might involve multi-loop strategies and include deployment of moon subprobes. Subprobes can use a main orbiter as a relay for communication.
For the inner planets the backbone system can be used. For the outer planets power is as usual a thorny issue, and arises even for the outbound flight. One possible solution is a small stopping/restarting nuclear reactor, a heat engine, and a battery, on the main probe; and RTG's on the subprobes.
Safety considerations for the reactor can be addressed as follows. The nuclear fuel is removed from high strength containers only once it is in solar orbit; and only then placed in the reactor. "Hot" objects always end up in orbit around an outer planet, or in interstellar trajectories.
Use of rockets and aerobraking, nuclear ion propulsion can be avoided in probes such as described above. However, it might have uses.
Sample return might be an unnecessary luxury, given the equipment which can be sent to the planet. Strategies for accomplishing this can be considered, though. These might involve putting gathered samples into interplanetary orbit using an ion thruster on the main probe(gathering might involve rockets and RTG powered ion thrusters). The interplanetary orbit is "Earth crossing", and the samples are retrieved by SIP.
Nuclear ion propulsion might be useful for a heliopause probe. If lifetime is an issue, staging with multiple engines can be used.