I have serious doubts as to how well this fantastical notion of mine would work as a reasonable lightsaber blade however I thought perhaps maybe I should input something different than the standard, it's a plasma blade answer.
My idea is to make the blade out of a powerful electron beam. Electron beams are beams of electrons fired out of a particle accelerator of some sort.
Now to figure out a way to miniaturize an accelerator that small is an exercise for the author of the story, however electrons do have the advantage of requiring less energy to accelerate than their other particle beam kin such as proton and neutral beams. Other forms of particle beam would either be too complex or power hungry, or too long range.
For example, a proton beam would require the stripping of protons and electrons from hydrogen gas, and neutral beams would need both a proton and an electron beam combined. Whereas electrons are abundantly available in the form of electricity.
Electron beams have high dispersion and are generally quite short ranged as a result. Perfect for our blade as it would mean that the particle beam would not extend too far from the hilt and could be focused/defocused to achieve longer and shorter ranges.
However there is the issue that natural dispersion might not be able to keep the blade from extending too far from it's source, but since electrons can be manipulated by magnetic fields you can perhaps even cause the beam to loop back to its emitter, and be recycled.
Now assuming you work out all of the kinks of the design the weapon would have the following properties:
-Its damage would be far more devastating than any mere plasma beam as the electron beam would be bombarding it's target with a stream of high velocity electrons that not only transfer high heat to the target potentially melting it, but also the millions of tiny electrons slamming into it's molecular structure at relativistic velocities would also cause kinetic effects at the microscopic level that would blast a hole through it. This has the added effect of being able to ablate away quicker at even the most heat resistant materials.
-As the electrons collide with air molecules they'd create light and while that wouldn't look like or resemble a normal lightsaber, they'd arguably look cooler as they'd look like crackling blades of lightning and most likely sound like it as well. However also the sound of the accelerator in operation could generate a low hum. Keep in mind the blade would only glow in atmosphere.
-When fighting with these blades assuming the electron beam generated by the blade is strong enough the blades would repel each other. This is because particles of like charges repel each other. So two electron blades would bounce off one another.
-They are retractable, because you simply need to turn them on and off and they can be deployed in seconds. Provided the power source can discharge quick enough.
However they have two huge disadvantages:
-they emit deadly radiation. When the electrons strike an object they do it so forcefully it emits gamma or x-rays. This could kill the wielder if they aren't properly protected, and it would tend to fry any electronics it got too close to. However, I'm not certain how bad the dose really is, so maybe it might not be immediately fatal but cause cancer in long term.
-It can be redirected by magnetic fields, meaning that an object shielded with a negatively charged field could deflect it. However, it isn't exactly practical to walk around wearing a magnetic field harness strong enough to stop something if this power level, so not likely to be exploited.
To get around the problem of the deadly radiation you could potentially assume that the people wielding it are descendants of space colonists who were genetically modified to withstand this sort of radiation. Or it was developed by a species who wasn't overly susceptible to it. Or perhaps have the wielder wear a special protective suit.
Or perhaps you could make it into a plot device, that the hero only draws their weapon in time of great need because it takes a toll on their health.
However if anyone knows a way to keep the wielder and bystanders safe from radiation from this weapon. Let me know.
You could perhaps use insane magnetic fields to control the magnetic spin moment of the electrons causing them to reflect any harmful directly back into the emmitter to be absorbed by a small standoff plate of heavy metal. But this would need insane power budget and could potentially not be workable as it hasn't ever been done with electrons. However if done it could reduce the radiation output to sustainable levels safe for creatures nearby.
In all if a few fundamental challenges are met it could work as a surrogate for a lightsaber.