Machines have slots (battery slot can be used for this, or some other slot or slots), which take special modules that alter this machine parameters.
Things that modules can change (usually combining two or more effects per module):
1. Energy consumption. Usually increasing it, as a "pay" for some positive effect.
2. Working speed
3. Decreasing resource consumption (example: three iron dusts instead of two from one ore, it 3x time and 10x energy consumption)
4. Extra capacity for "in" or "out" slots
5. Change production type (example:refined iron instead of iron, or iron instead of iron dust)
6. For energy units - energy storage size or energy output.
7. Better effectivity for one particular type of resource (example:generator giving 50% more energy per fuel, but not accepting coal, wood, etc at all)
8. Reliability. Either increasing it (for nuclear reactor), or decreasing, adding for other machines change of "meltdown".
Purpose of this: "sink" for extra energy and material in endgame, more variety in game-play, with hopefully not much programming effort or resource (cpu, ids, etc) needed.
