A 20 year mission to Pluto would be unsuccessful without cryogenic or otherwise science-magical-sleep system. I cannot imagine a single set of humans who could spend 20 years together, without any way to leave the small space of a ship, and not absolutely lose their minds.
That long of a trip means we just end up sending absolute lunatics and/or small pieces of chopped up lunatics after the inevitable murders.
The ship would need to be very large - artificial gravity and adequate shielding for radiation and impacts would need to be provided, as well as an insane amount of redundancy and survivable abort routes. Attempting to do it with technologies we have, even with infinite funding, would be next to impossible without incurring in unacceptable risks.
For some time an assumption - a rule of thumb of a sort - was that chemical rockets can get people comfortably around within Mars orbit, and to get further away we need something better. That "something" is likely not solid-phase nuclear engines, with Isp about twice as good as LOX/LH2, and probably not gas phase nuclear engines... but thermonuclear engines, which get closer to feasibility with advances in thermonuclear energy field. A thermonuclear engine could be more attainable than a thermonuclear electrical plant, and cheaper LEO access could be a big part of enabling technology.
Having engines like that we may talk about much faster trips to Pluto and back.
