My suggestion is to find a build palette.
Choose one block that is like your signature block, something not normally used. And start with that.
Then decide on the secondary blocks. These would be the rest of the main building. Like stone or wood etc.
From there, the world is open to you.
Hopefully by choosing a main accent/signature block, this will give you a theme to work from.
Another option would be to thing of your favourite movie and go from there. Be it sci-fi, westerns, futuristic, modern, real life etc. this might help you think of a theme to work along,
Then jump in creative and play with patterns and shapes.
I could just say build an underground sewer system of a post-modern warfare/apocalypse city, but that won’t help you in the future. It’s always best to be inspired and build something you’re passionate about.
So go ahead and find that something. And, please post picture updates here as you progress along. I’d love to see what you come up with.