How do you see the opportunity (and complexity) of offering services/applications to customers in 3rd party networks (which are not my own network) using a service broker?

The short answer is “protocol mediation”.  This example illustrates where it would be useful:  Most mobile operators have services implemented in their network on a number of SCPs and these usually utilise standard INAP or a vendor-variant of a standard protocol.  When the user roams to a foreign network, the foreign network communicates back to the home network using CAMEL as this is the standard.  This means that the roamer cannot use any of the normal services that they normally utilise.  If the home network has a Service Broker, it is able to provide protocol mediation and access to the INAP and INAP variant services whilst the user is roaming.