First off: I like the idea of not all of the changes being beneficial. I think it may be fun to expand on that and make it so the accidents can either be cleaned up (costing resources, putting the planet back in order) or incorporated.

Say your planet has the random "science lab unleashes biological horrors" negative event.

All your planetary troops die, a few pops die (should make sure not all pops can, to avoid depopulating the planet) and you are now sitting there with a science planet with pretty hefty biological horror penalty (- hab, happiness and productivity, as a bunch of the planetary effort goes towards penning the beasts in and keeping your eggheads safe - which can't work because they're afraid of facehugger crabs lurking under their desks or in the closet).

You can now either: Pay a hefty amount of money to prepare for an expedition to find and exterminate the monsters, costing credits to pay hazard bonuses to soldiers and get equipment, with a lengthy period of maybe half a year during which the planet produces little to nothing as everybody works towards killing what they made.

OR making this catastrophy into a virtue by herding the things into one or two tiles of "bio monster habitats" that give a bonus to touching tiles for society research and a planet wide bonus to credit production due to tourism and visitors (think Jurassic park with random tentacle monsters).

Option one would be cheaper and faster, ending you up with the planet you had before. This would be the best result if the things escaped from one or two labs on your otherwise mineral oriented planet, for example.

Option two would be great for society research if you want it, or for credits if you do not, but would cost more to implement.

If you want to make the event more badass (or if you're less into research and more into military might) you might enable a wrinkle where fanatic militarists can weaponize the beasts, getting the Xenomorph Army and Xeno Cavalry technology options + 50 % of the progress towards researching it, but needing them to let the creatures run wild and kill the pops.

The planet will turn inhospitable (toxic athmosphere, changed by the creatures so they may live there better) but you can now harvest beasts to domesticate for your wars (you just build them on planets, but the lore would be that when you produce them, transports fly over and net a couple thousand of the uglier specimen to unleash on poor xeno pops. :D)

Not an option for 25 tile gaia worlds, but if it is a planet with only 8 tiles, 3 of which are blocked by something you can't get rid off and that you only had 45 % habitability for anyways, it might be an attractive alternative - just don't forget to build an outpost or you'll lose the border, unless that too will be implemented in the event result. ;)

I do really like having negative things, and your ideas are great. Currently playing with how to implement them though. Ideas on how to balance them?

The rule of thumb should be that there aren't too many of them per planet type, so they don't start becoming annoying instead of interesting.

(Just imagine having 60 planets, 10 of which are special, all of those 10 spamming events every couple of years - it'd eat massive resources and distract you, not good).

It may be best if you write as many events as you think are plausible and fun, give all of them the same % chance to be chosen from a list and lock the amount of events that can happen to 1 or 2 per planet.
This is also necessary so the planets don't become ultra powerhouses later, assuming that you take all the best choices to solve the events.

Another rule of thumb I'd implement is to make sure that the debuffs apply only to the planet and not to the empire - this will prevent planets from becoming so bad that you'd rather gift them away or bomb them clean rather than deal with the annoying modifier of a botched event.

The rewards should also only be as big as the debuff (meaning if you have an event that spawns these xenomorphs, they shouldn't just give - 10 % happiness until solved and + 20 % social research once solved - that'd be too much reward. Equally bad if they make your planet living hell but only give + 2 % happiness when solved, of course).

As to resource costs to solve them: I'd personally attempt to tier them. You can either go clever and give the player a description of possible solutions and make some of them obviously smarter than others, regardless of cost, or just go for tiered costs.



An example for tiered cost result:
Bad event happens: pay 100 minerals to solve for no bonus, 500 minerals to solve with a small bonus, 1500 minerals and the loss of a troop transport/military ship/science ship for a really gud outcome ...



An example for "work smarter, not harder":

Pirates steal around your commercial planet! - 10 % credits as they raid trading ships and make space unsafe around your major trading hub planet.

--- Pay 200 energy to pay them off (bad choice, they'll be back for more and might not even stop stealing from you inbetween payments)
--- Send troop transports to provide customs patrols to scare them off, pay 500 minerals (you lose the transports as permanent stationed troops, the planet gains a + 10 % energy credit modifier due to safer trading attracting more traders/businesses)
--- Send your military fleet to catch the pirates! (you might wanna make this a 50/50 one, 50 % chance to spawn one of those pirate cut-throat fleets to duke it out with your fleet with the same modifier above when you win and maybe some medium difficulty military tech unlocked from the wreckage because you had to fight, 50 % chance that the pirates will offer to work for you for a payment of 1000 energy credits, if you hire them, you gain a flat income of 50 energy credits per month for the planet "Privateer modifier", making for a good permanent and immediate energy boost)

I have a suggestion for NixBoxBones idea for the biological horrors event. You could make every tile with a science lab to a tile blocker with the horrors hoard/brood mother/hive or something like that and all the adjacent tiles to titanic lifeform tile blocker to simulate the raiding horrors(as paradox does in another event) and the planet gets the "Hostile Fauna" modifier(-10% habitatability -5% happiness +20% society research). You get options depending to the govern alignments, with different outcomes. For example…

For everyone: the cheapest, removing "Hostile Fauna" modifier and giving you the tech to remove titanic lifeform tile blocker and to remove the hoard/brood mother/hive.

A very society research/material heavy and troops costly event chain to remove the the "Hostile Fauna" modifier, titanic lifeform blocker and replace the hoard/brood mother/hive with…

mutant landfill(+8 society research) (see stellaris wiki)
OR
Herded horrors (titanic lifeform blockers) which give adjacent society research +1 and allow to recruit a new Unit which is limited by the number of herded horror tiles and very strong in moral damage. (there is an event I don't know the name; you get titans so to speak with much HP instead)
OR
a Monument is granted (like those nice art pieces you can buy) which grant the planet it is built on +5% happiness, -5% ethic divergence and +20% moral damage for all recruited army's AND the planet gets the same effects, but as a modifier.

Make that last option +20% moral instead of moral damage. I gues that makes more sense...

Currently messing with negative modifiers in my dev code. I really appreciate the input! The thing right now is that I don't want to mess with ethics convergence or factions because of the upcoming update. Will do as much as I can without