Looks like there will be a decent launch window in the 2030s that could possibly arrive in 12 years, but it would be a flyby. As noted, there are some problems gettting to stick around and orbit. I remember that when New Horizons was launched (going from memory), if the first launch window didn’t work, then the next launch window would be about a year later. If that one failed, then they would lose Jupiter, and to launch withouth Jupiter would tack on about another five years to the journey.
Fun speculation, but there’s just not enough interest in Neptune or Uranus or KBOs, other than from science nerds, to get another mission going in anyone’s lifetime.