Allow subscribing/unsubscribing to attributes on existing channel connection
The channel/websocket feature of PI Web API would be more useful if it was possible to subscribe/unsubscribe to attributes on an existing channel connection. Currently you have to open the connection with all the WebIds you want in the resource URL. If you want to change which values you are receiving, you would have to close the connection and open a new one. Adding this new feature would also solve issues with maximum URL length limitations.
This is an issue we definitely need to address for both the reasons you suggested. We should be able to add and remove attributes while a channel is open. We also need a mechanism for signing up for large numbers of WebIds when the list would be too long for a URL.