The Foundry Ontology is a powerful tool and one of THE selling points of the whole Platform. A lot of capabilities are developed that require the addition of separate Ontologies or facilitate those. This is great to share governance by assigning ontologies to different domains.
The main restriction or tradeoff that comes with this is that you cannot define Link Types between objects from different ontologies. Therefore, I suggest to add another LinkType category called “External LinkType”. This external link type could link to other ontologies and maybe even outside the platform and would be the basis to scale and orchestrate ontologies within Foundry but also connect them to other things.
From a technical perspective, I can imagine that link types are currently within ontologies because they need to be indexed. Keeping this separate category can come with limitations (performance / flexibility / availability) which one would deliberately accept. However, the benefits overweight in my opinion since things like master data systems / ontologies or common knowledge bases could be easily integrated into the different world views (ontologies) of respective domains.
My motivation regarding this primarily comes from the semantic web technologies where a relation is defined by referring to an IRI which is globally unique and also valid as a reference to things outside of the actually ontology (e.g. in OWL language or SKOS).
I think this would greatly enhance the semantic aspects of the ontology, since links for object types or properties to commonly accepted ontological terms would be possible and create transparency for understanding the data beyond simple descriptions or groupings.