How about a “Bus” engine which would accept incoming signals from any track that is routed to that bus. The output of the track could be selected e.g. by long pressing the track number, under the move left/right buttons. The default option would be “Master” and if the user creates a bus, it would appear as an additional option. The bus name could be set automatically as “Bus 1”, then “Bus 2” etc, with the option to rename it.