Basically when night time comes there are a lot more enemies and they are stronger, possibly even unscaled to your level. predators actively travelling the commonwealth looking for a meal.
2) FERAL GHOULS
Ferals are common underground but finding them there is no coincidence. Its a shelter from outside to them. Radiation heals them sure but that sun must make them somewhat uncomfortable and irritable so they arent too many above ground...until nightfall.
Urban areas and the city will be crawling with them. I want actual hordes where you are so outnumbered you have to run or get to a safe-ish place to fight. Or just rock up in your best power armour with you fat man launcher.
3)
Nobody is safe.
Predators and prey. Groups of mods roaming and killing stags and anything else. Anything i dont care so long as it is enough to make even the best players be on high alert when night comes. Its just fun like that