I've always wanted to do a fully automatic city block, where each block produces exactly one resource, but get bored before that happens.
I'm currently working through seablock, and that's some crazy spaghetti. The limited space and the general lack of speed means that there is a lot of tearing down and optimizing my builds. In traditional Factorio resources and space is basically unlimited so optimization isn't necessary, in seablock a clogged belt means the entire factory stops running. So now I'm focused on efficiency.