I feel like research and technology fits under interesting world in that it provides depth to the world and a progression in terms of production. It can also be a management challenge not just in terms of allocating human (and other) resources but also effectively retooling your industrial and distributive mechanisms to account for changes in productive goals. I have a vague idea of a very opaque but organic tech system that could also supply both mystery and surprise.
Boats can also be used for world building in that they could supply a safe means of interacting with distant entities. I doubt Sumer would've had many Harrapan trading posts if they had to walk the whole way through multiple foreign Nations each way. Think also of (admittedly iron age) Phoenician colonies as far flung as Senegal and Britain in search of trade goods.
That said, my biggest near term desire for this game is more economic depth, especially more supply chains so that my empire has more reason to expand and my cities can be more specialized. And tin. :P I'm not sure it makes sense until you've fleshed out the city and especially intercity mechanics a bit more, as you are.
In fact, I've been quite happy with your current pace and direction of development. I'm not convinced that feature creep is really an issue here as much as the core concept being fertile to the point of becoming a distraction in itself. I think your current focus should be on breadth instead of depth, and definitely not too much on balance until most core systems are in place. So maybe no no tin for a bit...
That's just, like, my opinion, though. I've actually been planning to make another detailed post of suggestions. Perhaps I'll start working on that.
edit: I accidentally a word.