Personally, I also want "more harsh weather/planet". There's a mod called "Zombiome" in the mods forums that does some world destruction that might fit the bill, but I haven't played it yet because I have DMT issues (I'm not using Windows).
Anyways, Some things that I think would be neat,
should TFP or modders need some inspiration
- Chance of acid rain (there was a mod for "rain is acid" somewhere on the forums for an older version of the game). I think "chance" would be a better balance, but 100% acid rain would still be cool. Mods today can increase the rain chance so it would be tune able.
- Radioactive cloud "coming through the area". To stay less affected you'd have to dig underground and cover yourself with stone/steel (lead plates?). To pull something like this off I would think you'd like to be able to find/craft a radiation detector. Or possibly a "weather station" to tell you chances of rain, wind, etc. Super windy = radiation cloud likely to pass through. The radiation would be less deadly than the radiation on the edges of the map, so if you're trapped and cannot dig underground in time you *might* survive. There was a mod ( Darkness Falls) that had random permanent "radiation patches" and that was/is? cool. there were some POI's that were permanently shrouded in radiation and you had to take pills/get a rad suit to go into them.
- Flooding would be cool, but likely kill FPS...but ... maybe it occurs at late game stages if you turn it on ( because you can handle FPS drop) or maybe it also triggers when the game sees you have an underground base or spend a lot of time underground, and it floods right on top of you
- Earthquake/ground fizzure/sink hole: Likely could be done "slowly" for non FPS killing. Basically dig a big hold underground and let the blocks drop. Wouldn't be much fun unless it hit your base, you, or really close.
- Lightening: Another reason to not go out when its bad

Strikes could take out blocks, set fires, maybe long lasting blindness/concussion if you saw it happen near you.
- Forest fire: There's been some discussion on this in the forums, and it would likely be a FPS killer, but if a fire could "travel" from wood block to block... even if slowly, it would be awesome. You'd need a way to put it out though.
- From the "weather station" idea above: If you had it then the air temperature could be swung way high/low to make "dying quickly by heat/cold" a problem. There's not a way I know of to control this with basic mods, and another issue is there's not a way to "get away from extreme heat by getting cold" (like hide in a working freezer, craft/loot an air conditioner, which to me is a balance issue as if you're too cold you can build fires/forges to stay warm. There's not a temperature "insulation" mod or characteristic to blocks, so you can't craft a "cold room" in hot areas.
- Temperature gets hotter as you go down, colder as you go up (and darkness is completely dark when going underground): This would make it harder to dig to bedrock and survive, also living on mountaintops. With some of the changes mentioned I above it would enable you to craft a place to live to counter the temp changes by living in the extremes. Additionally: Maybe occasionally have underground things "spring a leak" and start flooding. Its not really "weather" but same principle of making it hard to find a super safe area away from the world.
- Underground "river": makes digging an underground base a little more terrifying
- "special": Anything you can think of that's super rare

Ball lightening, meteors, tornado, hurricane, etc. Its not weather but there's a plane mod that has the occasional plane crash and its fun. The crash doesn't take out buildings or anything, but that would be a plus. I'm imagining most of these would tear up POI's/bases a bit.