Yes, this is a city builder but city builders had disasters from the very beginning (first SimCity game).
I wouldn't like random meltdowns, but currently nuclear plants are way OP - they have almost zero disadvantages while generating huge amount of power. So while I think that random meltdowns are definitely bad idea there are some situations that should cause meltdown - like destruction of the plant in fire or having water shortage for too long (that would definitely cause meltdown in real life too, because the reactor is not cooled anymore). That would be fair because the meltdown always would be players fault (like neglecting water or fire coverage)
SC2K has another system in which nuclear power plant has certain lifespan (50 years) and need to be replaced or they will explode (of course you get newspaper notifications that the powerplant lifespan will end soon).
And while real life meltdowns would definitely render whole city uninhabitable Sim City meltdowns never destroyed whole city, they just exploded and randomly scattered radiation markers at the map that caused huge pollution. So similar system would be definitely possible in C:S.
Another disaster that I would like to see are of course riots, because now extremly high taxes, unemployment or skyrocketing crime cause no problem other than citizens moving out of town. And riots should start basically immediatly after you raise taxes (they could be small at first but the longer taxes are high the bigger unrest is) because you can exploit current system by overtaxing citizens for a while, which gives you huge money boost, lower them and then raise again in a few in game weeks without any consequences at all. Riots would definitely prevent that.
Of course these riot and crime related situations should be in a base game, because otherwise they would be easily avoidable by disabling DLC.
I hope that CO is reading and will implement at least some of these ideas, if not in this game then maybe in Cities Skylines 2