Suppose I'm not currently holding a sword, but my ally is going to toss one to me on their next turn. Can I ready an action to attack with the sword that I'm not yet in possession of, with a trigger of "when I acquire a sword"?
A similar situation arises with spells that grant the ability to perform new actions, such as dragon's breath. Can I ask my wizard ally to cast this spell on me and then ready an action to breathe fire with a trigger of "when I have dragon's breath cast on me"?
In general, the question is: if I am not currently capable of taking a certain action, can I still ready that action if I expect to become capable of it some time in the next round?