Modern technology doesn't change the major constraints of rocket propulsion significantly.
In the absence of a gravity assist, the most fuel-efficient route from Earth to Pluto is approximately a 45-year journey. Bringing that down to 9 years already represents a large investment in propulsion fuel; New Horizons set the record for the highest launch speed of a human-made object from Earth (~16 km/s); the Jupiter flyby increased its speed by another ~4 km/s.
If NH had an ion thruster capability similar to that of the Dawn spacecraft, with enough fuel for an additional 4km/s of maneuvering, it could have managed roughly the same travel time without the Jupiter assist, or, with both the ion drive and the gravity assist, managed more like a 7-8 year trip.
That kind of capability would come at the cost of something else; NH's very high launch speed was only possible because the spacecraft was so small (about half a ton), and the fuel needed for such an ion thruster would be about a third the mass of the craft. Either a much larger launcher would be needed, or the science package on the probe would have to be much reduced.