I agree with the Steel Mill option. All the routes I have seen are using the same DLS steel mill assets, since these type of assets are very limited. A modern mill with some visible loading animations of produced products would be fantastic. I also like to see the static assets that are also a commodity, whatever industry is chosen. The second choice would be a modern Oil Refinery.
Edit: I also would like to a modern Amtrak passenger route, Amtrak stations, yards, cars for commuters and long distance, sleeper cars, diner cars. Seems to be a lot of Amtrak loco's around, but passenger cars are very limited.
Above all, I would love to see a new load/unload Industry track that is easier to use, more user friendly and more user controlled. Currently, it will list every dang commodity in existence. It would be nice to click a box for only the commodities you want to see listed for the subject industry. Also, it would be really nice to easily be able to click on the Industry track after building around it to make commodity changes, without having to move surrounding assets away to access the Industry track.