A problem we're having now is that they want to give us new worlds, so they aren't accused of recycled material, but they already used the two best, most obvious worlds (Mars and the Moon) for D1. So now we get a moon of Saturn that 50% of people had heard of, and two tiny planetoids that 20% at best had heard of. BTW, not that it matters, but gravity on Titan is 14% of Earth's gravity. Io is 18%. Nessus has trivial gravity. A child could lift a centurion without much effort. You might get escape velocity by jumping. None would have an atmosphere that would be helpful to us. All would be extremely cold. Titan is not a "water world," but does have lakes and rivers of liquid methane. Io is a hellscape of volcanoes and sulfur. Nessus is probably an icy rock, that would be called a comet if it came close enough to the sun for the tail to appear.