These are public entities so they can't just take one another over. The citizens of the fire districts have to vote in a new fire authority as they are paying taxes directly to whoever provides fire service.
That said I would still be surprised to see any of that happen. The initial regionalization efforts made a great deal of sense. Many small fire districts of similar demographics merged together. But South, West, and Littleton are quite large as it is so I don't see how a merger would help. Eventually the organization would become too spread out.