The ability to create your own NEDs already exists and has been there since the very first versions. Note that for most cases you don't want to do that yourself - Cisco provides plenty of NEDs and creates new ones as needed.
However, there is one common case where a custom NED is interesting - NETCONF devices. For these devices it isn't really a NED in the usual sense since NETCONF is built-in to NSO but you want to create a package of YANG files that are loaded into NSO.
What 5.x does is simplify this process, in particular it allows you to have multiple instances of the same namespace. A common case would be if your devices have the same model but in different, incompatible, versions - today this would be a problem but in 5.x you will be able to handle it much more smoothly.
