I may actually be able to help with this. I'll need to talk to my co-author, as he does all of the programming, but this is exactly what our mod does. It generates structures, but divides up block placement into something like 100 blocks placed per tick. Since the code is already in place to break up the block placement, I imagine a special version could be released where it could be slowed down further (to maybe, 1 block per tick). If you have an existing structure you want to see built, we'll need a copy of the world file to make a snapshot, but this should be possible.
I should note though that the user can only create predetermined structures within the mod. Which is why we would need the world file if you have an existing structure you want to have built.
I think your mod would do the trick. I also thought it would need predetermined structures and that is okay for me. Only one problem: I don't have a structure at the moment. I first wanted to know if it was possible at all, before i started building. The second thing is, I want to use it for a YouTube semi-scripted series and the script is completed yet (because I wanted to wait untill I knew it was possible). So i don't actually know what needs to be in the building..
Is it a option that I contact you later when I know what I want to build and you have more time for the mod?
Do you have a page for this mod? It would be cool to keeping checking it for your updates :)
Of course! I actually linked it at the bottom of my previous post. The download page for the mod, and a link to the spotlight if you'd rather just see a demonstration of what it does. It's currently available in 1.10 and 1.11 flavors.
Currently, Prefab has a preview function that lets you see transparent directionless placeholders for the blocks (so it looks a little funny, but its more so you can see approximately where the building will be). The build rate for it is 100 blocks per tick, because all at once was causing serious lag in single player and server play. So I imagine we could throttle that back to 1 block per tick, which would put you right at 20 blocks per second build rate. Faster than a player, but not invisibly fast.
Feel free to check out the structures we already have in the mod, and we do take requests if there's something you'd like to see added.
I was actually going to mention the Prefab mod when I read the OPs post. I didnt realize the Prefab author was responding.
I am currently using Prefab in the modpack I am designing, and can confirm it does most of what you are asking for. The buildings in the pack right now seem more functional than decorative though. With the exception of a couple of the small houses having a cute theme, the rest of the buildings are pretty much made for a purpose. There are no big decorative castles, or spires, or lighthouses, or anything like that.... just functional buildings like Chicken Coops, Tree Farms, Monster Masher, etc... Maybe they can enslit the community to help them design some building templates for larger structures.
One thing that I think could improve the mod, is to have an option in the config that makes these structures randomly generate across the world... instead of only being available at player request.