Another Splunk app could be to perform a neteork capture on any machine. This could be machine local capture or app local or even a browser page local. In these cases,  the functionality is to sniff the packets similar to what Netmon,  Fiddler or Wireshark does. We can listen on the network interface and log each packet.  There's functionality available on windows to do that with the wininet library. We could even filter the packets per process. The application can be written in C# and with Splunk SDK library. All the packets captured can be a modular input to Splunk. The events written to Splunk can have the same format as the request response details on any one of the network capture tools. In this tools there is an option to interact and replay the packets or modify and sens a new one. In this case, this is a passive listen and log mode. The packets may be binary or text and we may choose to capture only text. We may have to switch the proxy for the duration of this application.  And the packets logged can have all the header information even if it is https.
What this functionality provides is a way to search the packets in a way like we can on Splunk with powerful queries that no other application has. It will also give us the ability transform data before it is indexed. Further more, the application could have a great deal of machine data generated over time. Where it differentiates from the existing Splunk out of box ability to log top and udp pack wets is its scope. The scope can be a machine or app or session. It should even be able to work on a headless server or on a device. In Splunk language it would be able to work on a single lightweight forwarder that sends events to an indexer from which search heads can pull results for the users queries.