I would find it useful to be able to:

• Make a dispenser dispense a specific given item. Eg. make a dispenser block shoot arrows on command (regardless of the items stored inside the dispenser).
• Similar to the first point, but make a dispenser drop a specific item, instead of using the item's default dispense behavior. Eg. drop an arrow instead of shooting it.
• When a dispenser dispenses an item from its inventory (BlockDispenseEvent), make the dispenser drop the item, instead of using the item's default dispense behavior.

I will prepare a PR for these.