Engine startup sounds would be great upon pushback or turning seatbelt sign on and shutdown sounds when turning the seatbelt sign off. An example of startup is shown below on a Boeing 777. Hear the startup sound from 2:25.
Another example of startup below is shown on an Airbus A321. Hear the startup sound from around 0:10.
An example of shutdown is below.
Maybe this could also be incorporated if an engine on/off button is added.