What about mine-carts with chests moving and dropping items using hoppers under rails (item then can be transported to the last dropping hopper using hopper/dispenser, etc methods)
Could be more than 1 layer or rails made such way that items could be dropped without hitting other levels.
Players will not know when the next dropper will drop an item as there is no specific time interval.
I think it will add some randomization.
I have a design for this