|a metric system can be based on readily measurable cosmic constants; albeit there may be complex-related co-systems|
We consider the most accurately known/knowable universal constants:
1. Cosmic constants, measured in our solar-system Earth-space near-vacuum space-aether
2. Separated by established theory (primary) versus taxonomic measurements (secondary)
3. All dependent on the cosmic eonic era: cosmos size, density, nearby masses, fields
4. Discovered equalities in theories (often asymptotic)
5. Basic mathematical constants, relating dimensions
6. Identified co-systems where dimensions are not simply related by the other constants
Primarity is almost variable itself: depending on what is known/knowable. For example, the speed of light is deemed constant over the frequency spectrum; but these are photons from electrons of known mass ... already we know light-speed varies through gravitational fields, even along a centerline,- and, how-much gravitational mass is in our cosmos, affects the speed of light; and local galactic density further adjusts that;-- but measurements taken, inside, may-so compensate the measurement-process (while thereby misjusting the speed of light across the cosmos outside our galaxy, probably a tiny amount too small to count, yet thereby a secondary effect on a primary constant)-... does a muon-emitted photon run the same speed? or a doubly-charged-[parton]-emitted photon (eg. helon-anti-helon co-orbit emission)? The mere fact, while unknown in measure, that the halves of a photon -transversal wave- stay together over cosmic distances, means there is some co-munication, suggestive of ordinary transversal wave process: therefor shallower "longer" waves, more compressive longitudinal speeds, may, be, faster: traveling straighter ... it may be in the photon wings' longer waves.
Just a few basic constants are needed, to specify all the dimensions of length and time ... even mass and temperature and electronic charge eventually are derivable from just a few basic constants. .... the major constraint being general usability - the major improvement being cosmic constancy (and the search for extra-terrestrial intelligent civilizations).
REF: values from physics.nist.gov (NIST Reference on Constants, Units, Uncertainty) CODATA