This may seem like an obvious question. Unfortunately, sometimes it does not have an obvious answer! If the objective is too specialized, then the PCB will not be useful for anything else. However, if the objective is too broad, then the PCB will be too bloated and expensive.
For low production and experimental PCBs, it is better to err on the side of being too specific. Design another PCB if necessary!
For really high production PCBs, it is also better to err on the side of being more specific. This has to do with the cost saving can be very significant if a PCB is more specific and smaller.