So i love building. Just the plain thought of it gets me excited. However, i always build in rectangles, T and L shapes, with only a small selection of combinations of blocks. I don't consider myself a bad builder, but if i get a unique idea i try really hard then i am out of ideas for a few weeks. And let's not even talk about decorating and adding depth, besides pillars and stairs i don't know how to add detail to houses. I want to start a new creative world and build a lot of houses wherever until it eventually becomes a town. I want every house to look unique really. I see everywhere houses in all kinds of shapes and creative layouts and wonder how people come up with such things. Only earlier i saw 10 house layouts i could never even think of.
So anyways, the main idea is how could i come up with unique layouts (horizontally and vertically) and find other combinations besides cobble/stone brick + any planks?
Do you like medieval style, with buildings created to fit a survival game, so that minecraft characters could use them?
Or do you like modern where passages are 10-blocks high (to increase detail?)
Or do you like minecraft-sized buildings but in a futuristic view, which may or may be not abandoned?
Victorian age? A large body of water in which you build green floating houses (water elves, maybe)? Ancient Rome? Arabian Nights?
After this choice, adapt the shape of the building to the style. Start easy, don't try to build an Empire State Building, a Pentagon or a Colosseum right away, maybe you village needs a church, which is probably cross shaped with a small bell tower.
Maybe you futuristic post-apocalyptic compound has an octagonal center building that, unlike the surroundings, is quite in good shape.
As someone who has trouble being creative as well, the best thing I've found is to use real life buildings as a guide, and to copy other people's designs to learn how to use them. Obviously don't pass the designs as your own, but from there, learn how to use the designs and concepts in your own ways. Soon, you'll get comfortable enough with the style that you can build it without any guides.
1. Challenge yourself. Put yourself a random constraint: build using only one chunk. OR build in non flat terrain. OR build using only grey blocks OR build using what you can fit in a single chest. Etc.
2. COPY. Don't be afraid of copying other people good ideas. Try as most as you can copying the whole build, but if you want, do it. The idea is to copy specific ideas. Do you like Grian's roofs? So build a serie of dirt sheds with his roofs. So you like someones walls? Make several walls, but just the walls. Copy ideas of corners. Try to repeat in different shapes the ideas you liked, and try copying different styles. That will help you build repertoire, and also, to start noticing what makes a great build.
3. Try different materials. You saw a nice wooden roof made with slabs? Mmm, maybe you can use quartz instead. TRY. Even if it gets ugly. Ugly builds are great. They'll teach you what you don't like.
4. Pay attention to real life architecture. To the details and the big picture. Look at this nice house you found, now look at the whole street. This other house has a nice window? How does that window goes with the rest of the house? You can do that even in films or series, just look at the builds and imagine them in Minecraft. Pay attention to floor plans. When you visit other peoples houses, pay attention to how the internal space is distributed, and all the different ways you can combine bedrooms, living rooms, kitchens and bathrooms. Pay attention to public places as wells. Like Palin mentioned, christian churches often have cross shaped floor plans. What about banks? Libraries?
5. Vary. Avoid making the same builds. So last week you built a big Medieval Castle? So now you build a small suburban house. Next, make a factory, and then, a pier.
6. Reform, and rebuild. So, you liked this nice cottage you built? Ok, leave it there for a couple of weeks, maybe months. Then go back at it, and think what would you change. Change it. Don't be afraid (maybe backup first?). Ok, so you really liked it? Fine. Open a new world, and try to rebuild it, relying only on your memory. No prints, sketches, plans, nothing. Just memory.
7. Exchange worlds with friends, or download them. Then apply number 6.
8. Make horrible builds, on purpose. Like number 3, you'll learn a lot about what you don't like. And who knows, you might even be surprised by something you though it would be horrible and you ended up liking the way it got. Who knows.
FInally, 9. TAKE RISKS. Like a friend of mine says "it's just a block building game, anyways". Nobody is going to die because your build is ugly, or poorly designed, or flat.