It's going to be another year before all of the data that New Horizons gathered is finally transmitted to Earth, but based on current science return, does what's been received so far warrant an advanced orbiter with multiple flybys mission that would navigate the Pluto-Charon system for an extended period of time, the way Cassini is navigating at Saturn?
We simply don't have the launcher system to accomplish that (see below).
That means a mission that can achieve Pluto orbit insertion (POI) after being launched from the Earth on a single rocket... and take less than 10 years or thereabouts from launch to first scientific results. ESA's mission study from Mark Adler's answer had transfer lasting 18 years. In 18 years Principal Investigators will most likely go senile, retire, or die, and their replacement will lack critical knowledge to troubleshoot payload instruments. (See also this question for more information on this very problem)
An orbiter mission is only practical with high-performance propulsion at Pluto. For reasonable trip times, as described below, the encounter velocity at Pluto is about 12 km/s. Pluto does not have enough atmosphere for aerocapture, and its gravity well is of low potential. Therefore, the entire encounter velocity must be removed propulsively. SEP does not work so far from the Sun, thus unless nuclear electric propulsion is available, one is left with chemical propulsion (jet velocity about 4 km/s, optimistically).
The practical mass ratio (total mass/payload mass) is about 40 to 50. A reasonable mission spacecraft mass is about 500 kg, leading to a trans-Pluto mass requirement about 20,000 kg. Not since the Saturn V has the U.S. possessed such a launch capability. Nor does any other nation presently possess such large launch capability. NEP is the only propulsion option that can reasonably perform a Pluto orbiter mission.
NEP - Nuclear Electric Propulsion.
SEP - Solar Electric Propulsion.
In addition, you have to choose among scientific missions to maximize their marginal scientific results. Titan, Enceladus, and Europa all look much more promising #, and are much easier to get at with reasonable payload masses.
* Despite the source being 13 years old, nothing has really changed as far as heavy launchers are concerned. Sigh. We don't have nuclear propulsion, either.
# Liquid something yadda yadda wanna see clash of the titans and the snakes at enceladus ALL THESE WORLDS ARE YOURS—EXCEPT EUROPA ATTEMPT NO LANDING THERE.
There are definitely some interesting questions resulting from the Pluto flyby, that I'm sure scientists would love to study. However, there are a number of objects that are higher in the priority to study in the outer solar system. Here is what I think the expected priorities would be, for an orbiter class mission:
- Europa, with other observation of Jupiter's moons if possible
- Titan (Might require a dedicated mission just to study this moon)
- Neptune (Triton in particular is of interest)
- Flybys of other major KBOs
The bottom line is, while Pluto is a very interesting body, it is only a small body, and not of particular interest such that it would justify an orbiter, particularly in terms of how difficult it is to get there. It is with little doubt the most fascinating KBO, but I just can't see an orbiter for some long period of time, owing to the difficulty in achieving orbit, as well as the fact that there are fewer bodies to study than the moons of any of the giant planets. Mars is more interesting, and closer. Pluto is probably the most interesting rocky object in the outer solar system after Titan, Europa, and Enceladus, but it still doesn't quite beat those 3 objects.