There's a difference between replicating, and creating. I expect everybody in their Blockland lives does a fair amount of both. Although I think the majority just create from scratch with imagination.
Personally, If I join a server, I'd see what everyone else is building, and then start work on my own while trying to better everyone elses designs. Not saying I always do, but at least it fits in well. When I'm on my own server I'll often create what I've been thinking about recently. I may have seen something on these forums, seen something on TV, or may even create an object in a large scale. The possibilities ARE endless, you just need to broaden your mind. Being unable to think of an idea with millions of options to you is a bad sign, and if you dont like what you build, then go ahead, replicate. Once you have copied enough things, you should have a good grasp of what techniques and styles you like, and will build quicker, and at a higher quality.
If you want to follow the last method, google "historic buildings" or be more specific and type things like "greek temples". "egyptian sculpture" and so on.
Best thing to remember is, nothing you create should be awesome in your eyes, but other people may love it. Trick is to be modest, but always seek to improve, and just don't stop building. Ideas will come, possibly while you're building something else!