That's a pretty sweet setup, with that empty lot next door.
New next-door neighbors are always a roll of the dice. I would err on the side of being left alone. But it wouldn't hurt to chat with your other neighbors, and let them know that additional space is a big deal to you, and part of what made your original purchase attractive in the first place. That you can understand why they might want to sell, but hope they understand that you aren't just trying to be a dick about it, but that it's important to you.
Oh, and don't ever piss off a neighbor who's a cop. If they want to make your life miserable, they can, and will. Most won't care, but the one who does can make you his new hobby.