Yeah, I can't find the link but I thought I heard something about recent efforts to ramp up USA Pu238 production for imminent deep space probe fuel. If I can't find it on Google, was it a dream? Ugh.
But I totally agree with you. We're long overdue to constrain our outer solar system characteristics, daunting a task as though it may be. Pluto and Kuiper Belt data (though regrettably single-point and in situ) will trickle in over the next few months/years, but we should have already launched another probe. Cannot believe Cassini is still going strong... that's a Pu238 powered craft.
As I'm sure you know, the main issue with orbiting Triton is reconciling the amount of fuel needed to brake for orbital insertion vs. the mass of the spacecraft at launch. When relative spatial scales are so large, but we want to reach them within the lifetimes of the people who built the instruments... yeah, therein the problem lies.
The most interesting thing to me about the Pluto data so far was the discretized atmospheric refraction layers. It should only be a matter of several months (or a couple years at max) before planetary atmosphere models can reproduce this observation.
The converse inner-solar-system mission launching maybe/eventually/finally in 2017 - Bepi-Columbo.