This is going to be a hell of a ride.
Involuntary implants for behavior control of violent criminals, or just hackers taking control of regular implants, or worse still ... advertisers
It is not hard to come up with plenty of dystopian scenarios involving this sort of technology.
My thoughts are that each brain has their own neural network that is unique to the person. This would mean that each neuron, despite similar/same location, will respond uniquely, as our personal neural networks were trained separately (across many years/decades). While I believe it may be possible to train a system to output stimulation to invoke a response, it would be susceptible to the same issues as simple password hashing (that is, while you could break a single hashed password, it would be computationally inefficient). Ignoring quantum computing theory, I think this would then make developing a system that can accurately control someone in LONG sequences would be difficult.
stimulate_purchase_region_of_brain(customer_id);Imagine the gap between rich and poor to be muddled with an arguable gap between human species.
Imagine the very real arguments about what consciousness is, what a person is with relation to a self spreading to include the machine hardware, perhaps entirely.
Once two-way communication is achieved, there will be huge leaps in what people can do in their lifetimes. Doubly-so if we figure out how to improve our perception of time and turn ourselves into high speed cameras. We'll have more thoughts per hour, while having access to all of human knowledge and be able to run complicated computer sims (games or actual physical work sims) and do accurate calculations and quickly design stuff with CAD software.