The problem with Tau is that it's already used for a lot of other things. I like to use π with a line through it for 2π ("pi cross"). But that's less likely to catch on since Tau for 2π is already well known.
I think the various OBOFoundry ontologies are at different stages of maturity. One of their design principles is interoperability, but I'm not sure how often one is able to reason effectively across ontologies unless great care is taken to ensure that logical axioms are sound.
I'm involved in a project[0] that has an aim to provide better data integration by using OBOFoundry ontologies, but it's been a challenge in practice to merge the software with the ontologies in a coherent way.
There are a few which are really great. Basically my shortcut is Chris Mungall involved then it is logically and biologically ;) sound.
While we at SIB are more towards the RDF/SPARQL part of the spectrum (https://edu.sib.swiss/course/view.php?id=440). We do use OWL and obofoundry projects like Uberon and GO. For UniProt I took a lot of pain to make sure it is really compatible. We did the same for Rhea and ChEBI. This has paid of handsomely in new query capabilities.
There's an interesting clojure library called 'Tawny-OWL'[0] that is designed for building ontologies like this. It allows one to define a set of entities that follow patterns and logical axioms.
The OBOFoundry has a set of principles[0] that not all ontologies conform to. An ontology project can register with the OBOFoundry through their GitHub site [1] if they follow the principles.
It is a combination of licensing. Snomed-CT is there in part via ULMS. But these days there is an official Snomed and Loinc via FHIR maintained by their hosting organizations.
