Here are the presentation slides for the Uranus Pathfinder mission.
There is an entry for the New Horizons 2 proposal here:http://en.wikipedia.org/wiki/New_Horizons_2NH2 was a proposal for a second mission to a Kuiper Belt Object. Essentially it would be a backup for New Horizons. But it would fly past Uranus on its way to the KBO. I have attached the short NH2 proposal. Also attached is the external assessment of NH2 done in March 2005. The latter report quite clearly stated that NH2 could not be done because there was no additional RTG available for the mission by the launch deadline. But it also pretty clearly stated that the mission was a bad idea. The politics behind this is rather interesting, and if you compare it to what happened with Mars 2020 in 2012 you will see a lot of parallels.
From what I understand, NASA hasn't abandoned ASRG. It's just not on the fast-track for introduction to an operational system. Like many projects at NASA, there are still funding streams for some work on it, and work will continue on the back-burner.
Which is an especially interesting "what-if", because the nominal target was 1999 TC36. At the time it was thought to just be a wide binary system, but subsequent imaging has shown that it is actually a hierarchical triple system, with a tight inner pair and a third body on an eccentric orbit that rapidly precesses. It's one of the wackiest minor planet systems in the solar system.
One of the problems with extreme outer solar-system missions is the necessity (when launched with EELVs) to make multiple passes of other planets to build up velocity. In one of the papers referred to in the thread, it was mentioned that a conventionally-launched Uranus probe would be loaded down with heavy thermal protection for a Venus fly-by.Could this be a useful application of SLS? How much could SLS with the as-planned DUUS throw through a direct TJI or TSI to slingshot towards Uranus?