- Produce something common (like circuits) and only uses raw materials, or other common items produced by similar blocks
- Repeatable: this is the key thing. The block should be repeatable either horizontally or vertically, so if I need more production, I could just stamp the blueprint at the end of the row (or start another one, if there is no more space, or the previous row is getting too long). See example below.
- The facilities in the block should work close to 100% if there are enough resources
- It should be as compact as possible. This is not really a requirement, since in most cases there is enough space, so it doesn't matter if the block is one tile taller. But I think it adds a lot of fun to the designing process. This is the kind of challenge I was expecting from the space platforms, so I'm really sad it is not going to happen (soon)
Here is an example, an advanced circuit producing block with the marathon mod:

This block is 11 tiles high, and 11 tiles wide (well actually it is 12 tiles wide, but if you repeat it, then one block will take only 11 columns, because there are overlapping parts)
The facilities in this block:
- The four furnaces produce 1.58*4=6.32 copper plates in a second (when running on 100%)
- The assembly machine in the center consumes 6.25 copper plates in second, so it matches the furnaces pretty well. It produces 1.25 wires in a second
- The two assembly machines at the top consumes 0.46875+0.78125=1.25 wires per second, matches the wire production
- Basic circuit and plastic consumption is the same: 0.625 per second. Basic circuit comes from another part of the factory and produced by other similar blocks.
The plastic production of a chemical plant is 2.5 plastic per second, which is too much for this block (the chemical plant would only work 25% of the time). So I decided to make another block, which contains four instances of the first block, and a chemical plant for plastic production. This block is also repeatable, but instead of horizontally, it is vertical:

Currently I have three rows of this:

If there are enough resources, this produces 0.125*12=1.5 advanced circuits in a second. And if there are more resources, and more demand, it is very easy to extend this with more rows.
One more example at the end, a block producing petroleum from crude oil and water:

Thanks for reading.