This is a suggestion only about graphics, not gameplay.
Currently, most industries look identical no matter how much is delivered or produced. But many industries have visible "wares" next to the building at all times.
For example, in temperate sawmill, you always see wooden logs and planks outside, no matter how much wood is actually delivered.
I suggest to extend the graphics of industries to change their appearence depending on ...
1) How much (and what) is delivered
2) How much is produced
In concrete terms, here is a list of "outside stuff" that is supposed to change in amount depending on how much is delivered.
- Temperate sawmill: Add wooden logs
- Paper mill: Add wooden logs
- Oil refinery: Turn off the "flames" if no oil is delivered
- Power plant: Turn off the steam and "electro" effect if nothing was delivered, otherwise use the current default animation
- Printery: Paper rolls
For example: For the temperate sawmill, if nothing is delivered, there are 0 piles of logs outside. If a bit is delivered, there is 1 pile of logs. If there is a high delivery, progressively add more "piles of logs", up to a given maximum.
Also, a good is considered to be "delivered" to an industry if at least 1 unit of it was delivered within the last 1 or 2 months (not sure).
And here's a list of outside stuff for the amount of production. It works similar to the amout of delivery. The following shall change depending on the output:
- Coal mine: Amount and size of "coal piles" (but always at least 1 pile of coal)
- Temperate sawmill: Add wooden planks (representing goods)
- Paper mill: Add paper rolls
- Factory: Add random objects on the concrete floor
- Food factory: Put variuous wooden boxes outside. Also let smoke come out of the chimneys if anything is produced
- Steel mill: Turn off the "flame" if nothing was delivered
- Paper mill: Add paper rolls
- Bubble generator: Increase the amount and speed at which bubbles spawn as production goes up
- Cola fountains: If there's a low production rate (<100%), reduce the "level" of the cola in the cola bottles. Otherwise, use the default image
- Sugar mine: Size of the big sugar pile on the tlue pile increases with production rate
- Fizzy drinks factory: Remove the liquid coming out from the faucet and keep the glass empty if production is 0. Otherwise, use the default animation
- Caramel mine: Size of the caramel pile (maybe 3 sizes: small, medium and big)
- Candy factory: Idk. Maybe turn on the light in the window if producing anything?
For backwards-compability, the new special graphics for the industries for e.g. "lack of output" or "higher output" should only be used if the base graphics pack actually supports it. I suggest to add default support for OpenGFX. But if the player uses a 3rd party graphics pack like the original TTD graphics, OpenTTD needs to fall back to the default graphics for that industry.
Overall, I think this graphics change will give a nice touch to the game because it makes industries feel a bit more "alive" rather than static.