It would be helpful if nodes and edges could belong to one (or many) classes. A class in this concept is actually a set of properties. If a node/edge belongs to a class, it inherits the properties defined in the class. The "is a" class association predicate could be also a property mapping discriminator.
Why would this be useful: let's imagine we are drawing a network diagram, where PC-s, switches and cabling is also presented. PC-s have names, IP address, OS and so on while switches have port number, management ip address, manageability and so on. A link can be copper, optical or wireless - any of them has a nominal bandwidth, bot optical links have media type, wireless has SSID property and so on.
( Please vote for it :) )