Providing the player with new tools or adjusting the game interface, without fundamentally changing gameplay.
Transportation of the player, be it vehicles or teleporters.
Augmented or new ways of transporting materials - belts, inserters, pipes!
New ways to deal with enemies, be it attack or defense.
New Ores and resources as well as machines.
Things related to oil and other fluids.
Furnaces, assembling machines, production chains.
Changes to power production and distribution.
More than just chests.
Change blueprint behavior.