That's great news! :D
I'm developing a blocky game engine myself, and I'm very excited about the possibilities of Lua scripting. Having it in Minetest would be just awesome. I'd like to share some of my ideas with you, hoping you'll find some of them useful.
Let's first answer the question from the opening post: I'd like to build turrets that shoot mobs that get too close. I want to hook functions to an NPC, so he talks to players that approach him. If he takes damage, it would be great if I could hook that as well to make nearby mobs turn hostile. I want to build a button or a pressure plate that also works with double doors. Secret bridges that materialize under a player's feet. Landmines you can hide under a block of grass or sand. Decorations.
Have a look
at this draft, perhaps this illustrates the idea better. The core engine won't have fences or doors, but you can define them completely through scripts. Yes, this means you'll have to build your own fence models out of 1:16 mini-voxels, overload the "on_place" hook, and look at the neighboring blocks to decide if you need to swap them out for different models so they connect properly. But with the same functions, you can also build doors, stairs, oil lamps, window shutters, a door bell, ...
Let me know if you're interested in coordinating our efforts in some way. :)