For missions that are not planned to be launched for 10 years that have their capabilities largely limited because of mass, I think we should just wait for technology to catch up a bit. We are on the verge of a revolution in advances for shrinking sensors and mechanical objects akin to the advent of computer chip miniturization. I think it is very possible that in the time frames talked about for launching outer mission projects, we could radically redesign the probes (massive shrinkage) to have nano sized antennas, molecular sensors, radiation sensors, and even nano robotic probes. Then we could get order of magnitude improvement of probe capability for the same cost or cheaper and possibly lessen travel time to the planets considerably (lower mass probes).

I really do have qualms for planning too specifically for mission launches greater than 5 years in the future, when the underlying techologies could be undergoing significant advancement. I don't think it is too radical of an idea that the appropiate time for outer planet mission is when we can build a probe 10x more capability for the same cost as the last probe. That way we ensure major bang for our bucks. Of course I would never have wasted as much money as we have in our white elephants of the ISS and shuttle so much more money would have been available for the science probes that have been the only things really advancing our extraterestrial knowledge.