Idea from They are Billions

[Factory Overlord]

One thing I liked about They are Billions is how dangerous it was to expand.
I am really not a fan of Factorio monsters. it feels more like spam than a threat.

It would be nice if the World space was more "Hostile" to the player and required the player to modify the world in order to expand.

Examples

Radioactive Sites - These are areas of the map rich with radioactivity ( possibly from illegal dumping from a previous factory owner ) - they required to be cleaned before buildings can be made in that area.

Parasite bugs - These are tiny bugs that will attempt to enter certain machines and lower productivity.

Exploder bugs - if they explode near volatile equipment can cause chain reaction

Corrosive Gas Site - Vent that spews corrosive gas intermittently. Requires researching Gas Plant - which can be placed over the vent which converts that toxic gas to useful materials.

etc

That way you aren't simply spreading and having aliens more of a random annoyance but - the world it self has different problems to solve as you expand.

[Cribbit]

Sounds like a good mod idea.

[valneq]

I'd like to have something similar, too! But I'd prefer it to be optional. Or as a mod / many mods.

Wild guess: something similar already exists as a mod but I don't know about it.

I do know about https://mods.factorio.com/mod/dangOreus
It drastically inhibits expansion by filling the entire map with resources and destroying every structure (which is not a miner) when you place it on ore deposits. The only way to expand is to actually mine all the resources and thus have empty tiles to build on.
The difference to OP's suggestion: dangOreus is uniform in space, while OP was talking about various localized hazards being scattered around the planet.

[ssilk]

There are some more mods (Rampant for example), which make that experience more dangerous. But I'm on the same side: From some hidden border on, it should be much more dangerous to explore. I really like the thrill which comes from that.

Edit: In my eyes they should be much more clever. They need:
- Finding the weak point and trying to go through it without distraction from weapons
- Avoid strong points (by going completly around the base for example)
- Try to coordinate the offense (all at the same time from different directions for example)
- Try to hit essential logistic (e.g. water for power)
- The stronger the player is, the more nasty they get. For example they will not be distracted from anything (weapons, players) until they have reached their target.
- More different behavior. E.g. they walk in a row, eating trees, sand, stone and shit copper ore.