Engineering accuracy.
Small changes concerning balance, gameplay, or graphics.
Transportation of the player, be it vehicles or teleporters.
Augmented or new ways of transporting materials - belts, inserters, pipes!
Trains are great, but what if they could do even more?
New ways to deal with enemies, be it attack or defense.
Changes to enemies or entirely new enemies to deal with.
Map generation and terrain modification.
Related to roboports and logistic robots.
Furnaces, assembling machines, production chains.
Changes to power production and distribution.