I have not been able to find mission proposals for dwarf planets in the Kuiper Belt. The first mission to take a look at any Kuiper Belt Object will be New Horizons. After its Pluto visit it will be sent to a small KBO; they have picked out some potential targets but the final choice hasn't been made yet:
Because the flight path is determined by the Pluto flyby, with only minimal hydrazine remaining, objects needed to be found within a cone, extending from Pluto, of less than a degree's width, within 55 AU. Past 55 AU, the communications link will become too weak, and the RTG wattage will have decayed significantly enough to hinder observations.
There are also proposals for more missions to KBOs.
Argo will combine a Neptune flyby with a KBO flyby. This was a proposal for the New Frontiers 4 mission, but has not made the shortlist.
At one point around 2004, a 'New Horizons II' mission was proposed as a contingency: it looked like there would be a shortage of Plutonium-238 for New Horizons, limiting its lifetime so it would only be able to operate until its Pluto flyby. The proposal was for a second, identical spacecraft to be sent (with a full load of Pu-238) a couple of years later for a KBO flyby. As it turned out, the Pu shortage didn't happen, and New Horizons II was dropped.
The two organizations with the largest planetary science budgets (NASA and ESA) can only afford a few planetary science missions per decade. Which missions they choose, depends on a number of factors. Many of these factors are infavorable of a KBO mission.
- The scientific community proposes missions, and NASA/ESA make choices between those proposals based on viability of the proposal, budget etc. For the well-known planets (e.g. Mars, Jupiter, Saturn) there's a large established scientific community that comes up with new mission proposals regularly. Kuiper Belt Objects are a new field of study and have fewer advocates so on that basis alone you get fewer mission proposals. If there's a huge stack of Mars missions and one KBO mission, statistics alone suggest a Mars mission will be chosen.
- Outer planet missions must use a radiothermal generator as a power supply, and there's only enough plutonium-238 available to fuel one or two missions per decade. This may have been a factor for Argo: it needed an RTG, and none were available for New Frontiers 4.
- Outer planet missions are long-term: New Horizons will take 9 years to get to Pluto, for instance. That's 9 years of spacecraft support, keeping the science team together etc. before you can start the actual science. In the past, this had to be included in the budget, making these missions more expensive and less likely to be chosen. NASA has changed that recently, though.
- To reduce the transit time, New Horizons was launched at a very high speed. This makes it impossible for the spacecraft to enter orbit so you have a flyby where all your science has to fit in a few hours of closest approach. So there's less science return than e.g. a gas giant mission where you can enter orbit and do measurements for years.
- Outer planet missions need a big, expensive rocket to launch them at sufficient speed, again making the mission more expensive.
- Pluto is the nearest large KBO we know of. Others are several times further away. They're also very far apart. You could try to visit multiple KBOs, but you'd again have years of transit time in between.