TL;DR
Inspired by other games, like Path of Exile or Diablo... 1, 2 or 3 months long seasons in Factorio combined with community map from reddit.What ?
Let me explain 'season' for anyone who haven't played PoE or Diablo. Basically it is 2-3 month long event. Every new season developers adding new content to the game. It keeps the game fresh. Makes people to come back to the game and try new stuff, solve different problems, compete against each other because everyone starts from scratch.How it possibly could work in factorio?
Everyone starts at the same map. By creating the game you have an option to make it singleplayer (SP) or multiplayer (MP). In SP maps no more players are allowed to play, whatever you do must be built by yourself.
Your map is on the list so everyone can join an spectate. You can sort it by amount of rockets launched so it makes it a bit competitive. That's the reason to have SP and MP as a two different categories, to make SP players fair chance.
Spectating.
Everyone can join your game and watch what you are doing. Host can enable/disable an option to communicate with spectators. Spectators can join the game even when you are not playing, it means everything will be frozen. I am not expecting anyone to sit and watch for hours. It is more like to join the game and check different players approach.
Season theme.
Map could be very different. From standard map to death world marathon, one big island, infinite amount of tiny islands with tiny amount of landfill ( or just make landfill very expensive), 20 tiles wide map etc...
We could also add modifiers, like 'hardcore' - game over if you die, x100 science pack, evolution beyond 100. Even twist game a little bit by adding small mods, or make only mixed ore patches etc.
Goals could be very different. Most rocket launched is the most obvious. Would be interesting to see a island map with limited resources and check who was the most resource efficient player. Or biter bitter battles theme map and check who will survive longer.
Why ?
I keeps the game alive? Bring new players looking for a challenge?I know there was or maybe still is a 'community map' thing on reddit. It was played maybe by 15 players so it wasn't that popular. Same story was in PoE which by default allow players trade between each other. Some people started SSF (Solo Self Found) on reddit. Wasn't very popular by that time until developers decided to implement that feature to the game