Jason Perry has a nice summary of the Io white papers at his website: Io Decadal Survey White Paper. There are actually two white papers, one which discusses science goals and one which discusses possible missions. Neither had anything particularly new, and Jason does a good job of summarizing them. The most interesting part of the discussion was the endorsement of the Io Volcano Observer Discovery proposal (~$450) and in the next breadth discussing New Frontiers (~$650M)missions to conduct essentially the same goals. My guess -- no knowledge -- is that the community is nervous about the mission fitting within the Discovery budget. The last budget estimate I saw for the Io Volcano Observer put it slightly over the Discovery budget limit. The key issue, I would guess, is likely the technological risks of the radiation hardening fitting in the budget with acceptable margins.
An Io observer mission requires a plutonium power supply. The next Discovery mission can include such a plutonium power supply (to test the ASRG supply) while the next New Frontiers mission will not (to husband dwindling plutonium supplies). If an Io observer cannot fit into a Discovery budget (or can, but isn't selected), then the next opportunity is the late 20 teens New Frontiers opportunity. That would put Io in competition with the proposed Argo mission to Jupiter, Saturn, Neptune, Triton, and one or more Kuiper Belt objects (as well as other good missions to non-outer planet destinations.)
The key issue for any dedicated Io mission, however, is the planned flybys of Io by the Jupiter Europa Orbiter (JEO). A dedicated mission would provide much better Io science through optimized instruments and many more flybys, but will the community accept a flagship mission to Jupiter and another Jovian dedicated mission (given the Juno Jupiter polar orbiter and JEO)? My guess is not. If JEO proves too expensive to fly, then I think that a dedicated Io mission has a good shot.
Io Science Goals White Paper
Io Missions White Paper