So far, all progress in the game is simply made to get better tiers of machinery and materials... This is fine, for now. BUT, what happens when I'm at the last tier? I just make more efficient machines for fun?
Some people might enjoy that, but I have another suggestion:
Let us pursue a high score of some sort with production. Something to do with all the excess materials we make, instead of just dumping it all into a box for later. One idea for this would be to assign a "score" to every single item in the game, that goes up with part complexity. This way you could measure your world's production capability per hour, and possibly compete with your friends on who has the largest and most efficient world!
We could also have a building that converts items put into it into some sort of score, like a market. Any items pumped into it are effectively lost, but give you some score, which can then be measured in score per hour, and can be compared to other people. With this simple building some people could already have a goal of pursuing higher scores by increasing the complexity and scale of their factories.
The most important part of this, is that more complex and harder to get parts must always be more efficient in producing a higher score. This ensures that someone won't get a huge score just by pumping a ton of raw materials into the market, but must instead build massive factories and infrastructure to maximize gathering of materials, the production of new parts, and the delivery of items to the market.