I so agree with you on Neptune's appearance being stunning but to tell you the truth, for some reason those outer planets seem a real waste of resources to go out and explore right now. Sure, eventually when we have really improved propulsion systems that can get us there in the blink of an eye it makes sense to spend time on them but right now I'd rather see those resources devoted to planets a bit closer to us. We've only touched the surface on exploring Venus. Yeah, it's a living hell but what about ballooning through its upper atmosphere. Mercury might be doable for a rover if it stayed on it's dark side or just at the divider between night and day. The probes to Jupiter and Saturn made sense because their moons hold great promise for possible life. I'm pretty much convinced anything out past there is pretty much just a tourist destination and will provide little in the science of how our solar system was created.