Pollution would be awesome. Have it as a sum of all machines running over time, when the level gets high, the sky starts getting clouded out, when it gets really high you get acid rain, which destroys the leaves of trees and damages entities out in it. Pollution of machines could be reduced with a catalytic converter upgrade.
Wouldn't it be somewhat redundant to make pollution reducing furnace addon for furnaces? That's why we got the geothermals and solars and stuff.
Though, having different fuels produce different amounts of pollution, as well as certain reactions producing pollution too.
So something like charcoal making in a furnace producing some pollution, and then burning it in a generator alone would produce more pollution than burning coal in a generator alone, as coal and coke are fairly clean fuels in comparison to charcoal.
THOUGH, there should be a way for radiation to be handled in reactors.
If there could be a radioactive waste item with a system to track where they are (who knows how that'd be implemented), they could have radiation leak into the local environment around radiation, and doing something like dropping it on to lava, cactus, or allow it to despawn, it should leak a truly substantial amount of radiation in the environment.
So you could bury it in a box deep underground to prevent it from instantly tainting the local area in radiation, like everyone usually does, but it'll still leak slowly in the area around it.
This is still assuming someone could get a system that tracks where items are put into boxes or placed and such, which is borderline ridiculous to figure out, and perhaps should require an entity based system. Piles of radioactive waste under your reactor that you gotta grab with a special item like portal gun grabbing cubes, and move it somewhere else? Perhaps.