Well, as I stated, what you'll be building will be feasible in an hour and a half. The reason why is because I'll be recording the entire thing, and then speeding it up so it looks time-lapse-y (or however your supposed to spell that). For example, some of my past thoughts for this were "lighthouse, medival structure (but that would require DokuCraft because medival stuff looks epic in it), and skyscraper" were all former ideas. Each of which could be build in an hour and a half/two hours ( I will be glad to extend the time limit if everybody agrees it needs to be).
Why I think this would be fun is because it would be awesome to see what kinds of things people can build on the spot-if you know what I mean. This way nobody has an advantage over anybody, and it will test you because you really dont know what your building. I promise you the first one will be about as simple as the ones I've stated before-it will be on a flat world, so yea I can't make it too complex.