There does not appear to be a thread about this proposed New Frontiers mission, so I am starting this one. If there is an existing thread that I am unaware of, then by all means merge them.
Argo is a proposed outer solar system multiple flyby mission, rather like an updated Voyager mission, but based on the New Horizons bus, and using a similar instrument suite. It would launch between 2017 and 2019, with either a Jupiter / Neptune / KBO or a Trojan / Saturn / Neptune / KBO trajectory. Even a combined Jupiter / Saturn / Neptune / KBO trajectory is possible.
Neptune would present a much different system than in 1989, as telescopic observations show a more dynamic Neptune atmosphere, due to the change in season, and much more of Triton and the other Neptune satellites would be visible (most of Triton's northern hemisphere was in darkness in 1989, but will be well lit in 2030).
The second big payoff would be the vastly greater access to KBOs (~4000 times the accessable volume of New Horizons), with several already-known large KBOs (400km diameter or larger) within reach. The objects reachable with Argo are expected to include:
18 cold classical KBOs (interesting because they apparently formed in situ beyond Neptune's orbit, rather than further inward)
40 KBOs with diameters between 200km and 400km
9 KBOs with diameters greater than 400km
several binary KBOs
plus the possible Jupiter Trojan early in the mission. The wide expected range of choices allows for the selected KBO to be of very high scientific interest (and naturally, follow-on KBO targets could be selected after the primary KBO target has been selected).
Typical flight times from launch to the Neptune flyby are about 10 years (Jupiter gravity assist) or 13 years (Jupiter Trojan flyby), with the large KBO flyby 2 or 3 years later.
Expected cost including launch vehicle (according to the linked pdf): under $800M with the following strawman instrument package:
High resolution visible camera: New Horizons (NH) or reduced Cassini heritage
Near-IR spectrometer: NH heritage
UV solar & stellar occ. spectrometer: reduced Cassini heritage
Far-IR linear radiometer: Diviner heritage
Magnetometer: replaces NH dust instrument
Charged particle spectrometer: NH heritage
Gimballed high-gain antenna: heritage radio science instrument
The big uncertainty at this point seems to be the availability of plutonium-powered RTGs by the time of launch.