It's a great idea to base a large project on a theme. The only issue is that doing so can obviously limit your design choices. Do you have a favourite style of architecture in real life that you can base your creations on? There are many styles to choose from and you can take inspiration from fantasy, history and even sci-fi. I would perhaps wait until you know more about 1.3 and the new materials that await us until you decide exactly what it is you want to build.
I shall personally be building my 1.3 world with the goal of creating a city/town in the style of these images, which show the colossal Albert Memorial Tower that was proposed in 1904 to be built in Westminster, London but was unfortunately never built.
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Whatever theme you decide on, be sure to gather lots of images as inspiration and try various block/paint combinations to get the exact look you want. If you do decide what theme you would like to try I'd be happy to suggest which blocks you could use.