Well I noticed there is no way for getting the event where a players' inventory is updated.
There could be plenty of reasons for an inventory to update:

1. An inventoryholder pickups an item
2. A player drags an item into his inventory
3. Another plugin updates the inventory (Adding item, removing item, clearing inventory)
4. A player drops an item
5. A dispenser equips armor on a player
6. A dispenser gives something to the player
7. A player drags an item into another inventory
8. Items enter a hopper
9. Items are transmitted from a hopper
10. InventoryHolder dies
    I suggest to put all these in an InventoryUpdateEvent, which would have the following arguments:
    Inventory getInventory() - the inventory updated
    UpdateType getUpdateType() - the way the inventory was updated (An enum with ways of an inventory being updated)
    InventoryHolder getSource() - who updated the inventory (I dont know if this should be an inventoryholder since it can be a plugin aswell)
    InventoryType getInventoryType() - the inventory that was updated's type

And another event, PlayerInventoryUpdateEvent which extends InventoryUpdateEvent:
Player getPlayer() - the player who's inventory was updated
And all the variables from InventoryUpdateEvent

Now, of course these are all listenable - I can listen for pickup, dragging, clicking, dispensing, death and all that stuff, but I cannot listen for another plugin updating an inventory. That is a major flaw in bukkit & spigot and I think spigot is the place to change it.