Mods introducing new content into the game.
New places to build more factories.
Augmented or new ways of transporting materials - belts, inserters, pipes!
Trains are great, but what if they could do even more?
Map generation and terrain modification.
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.