We at CELUM also put a custom model on top of JanusGraph that supports a type system and multi-inheritance for vertex/edge types.
The global scope of property key definitions forces us to define all properties' data type as Object as same-named properties on elements of different types might have different types (this also revealed the issue https://groups.google.com/forum/#!topic/janusgraph-dev/3KIDmHuTcwo). Overcoming this limitation should then reduce storage overhead when we can work with concrete property value types.
We solved the traversal-time schema enforcement by having a (compile-time) type-safe query language on top of Gremlin that also implements the type inheritance logic (Intro: https://www.celum.com/en/blog/technology/a-querys-quest). Type inheritance is modelled via additional properties. Soon, I will release a blog article that elaborates on one of our use cases and highlights the benefits of a strict schema and type-safety.
