One of the big problems with having a level system in trials, like standard skills, is that the approach is so different. Standard skill levels are only really possible because the terrain is 100% predictable and is the same everywhere in the world (ie. flat).
Much closer to trials are the grading systems in climbing, where you grade the route difficulty, not the technique difficulty. With experience on different routes, climbers get a sense for what constitutes a certain grade and then they judge the difficulty of other routes based on how a certain grade feels to them. Climbing has a big advantage over trials in establishing such a grading system because routes are permanent features that get climbed enough times and described enough in guidebooks to establish a consensus grade.
In the earlier drafts of the unitrials rules, (prior to about 2003), course setters were supposed to rate the difficulty of obstacles and establish point ratings, where harder lines received more points. We did away with this because it was hard to do consistently around the world. Now every trials line in a competition is worth one point- much easier to administrate.
However, one outcome of that was a difficulty rating system called the U-System, which was inspired by the V-System for rating bouldering problems in climbing. You can see it here on page 16:
The idea was to have an open-ended rating system with examples of different obstacles at each level. The idea is not to “accomplish” U1, or U5, etc. in the same way as standard skills, by doing all the example obstacles at a certain level. The point is that the rider uses the examples to get a personal sense for how hard a U3 level problem, say, feels to them. Then, when they are out riding something else (including obstacles you couldn’t possibly describe or include in a skill level), they can rate it’s difficulty based on their sense of the U system ratings.
This system sort of died because unlike climbing most trials lines are obscure and rarely done exactly the same way by many riders, and none to date are described in guidebooks.
But it would be a good way to communicate about trials difficulty in different places in the world, and to track the improvement of the sport over time. The advantage is that unlike competitions, where you can only look back and say that a certain person won, with a rating system you could look back and see the actual skills progression. For example in 2002 there were only maybe 2 or 3 riders in the world who could do U8 lines, whereas today there are dozens of riders who can do that.