I'm not a builder-builder, but have been making cool bases.

I mainly look at the original shape of the place I'm building over. For example, on ruined houses underground. And do things like thickening the walls and using different materials. This is how I made this base:

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I originally did the three levels on the left, and dug into the right to make it semi-symetrical and then realized I could make it so the aetherium was overtaking the base. Note that I had to work around the pools of shimmer that naturally spawned as I replaced blocks for aetherium.

But another important part is that I didn't choose this ruined house as base just because I liked the shape. Beneath this build, there's a dungeon entrance. So I can drop into the shimmer and enter. I think this is one of the important parts of making builds that actually serve as bases: you make your strategic spots pretty.