> the only important feature of the project management system was that you couldn't assign the same priority to two feature

That is a good idea for a project management system. Force ranking of priorities.

Even better is when you can only assign a priority once, ever. So if one thing was marked "urgent", nothing else can ever be "urgent" again. It can be "Urgent" or "double urgent" or "urgent for real this time", but not "urgent". Forces creativity and maybe even, depending on the size and business (as in "being busy") of the organization, the creation of whole new words after all existing permutations have been used, from which we all benefit.

For a novel problem (sub)domain that is great, if no prior optimization occurred globally dominant bottlenecks / inefficiencies still exist.

But once the worst bottleneck is widened to the same width as the second-worst bottleneck, you are from then on optimizing both until you reach the width of the third-worst bottleneck, and from then on you need to elevate all 3 to improve the situation, and so on.

to make it more concrete with an example:

you can identify that the friction on a bicycle comes predominantly from the front wheel, so you optimize the front wheel bearing/lubrication/... until you discover the front wheel has the same friction as the rear wheel bearings, so if you want to improve you'd have to improve both front and rear wheel friction, which helps until they have improved beyond the friction on the pedal bearings, from then on you need to improve all 3, until you discover the chain links became the friction bottleneck, etc...

I could see this working as long as it applies per-level & only takes into account direct siblings, e.g. every epic must have a unique priority, and every feature belonging to that epic must have a unique priority, etc.

Unless you can change the rankings. Then... Then it gets dumb real fast