In an ad for Borderlands 3 on Twitter earlier this month players spotted a shotgun with the damage type "radiation". In previous Borderlands games there have been various damage types like explosive and slag, but radiation is a new one.
Gearbox CEO Randy Pitchford later confirmed that not only is there a new damage type but it's replacing an existing type: "There is no slag", he wrote on Twitter, "we replaced it with something that actually makes a little sense. The new “element” damage type, which is like a nuclear radiation effect, has a similar effect to slag (increased vulnerability by those affected) and also has a damage-over-time component."
In Borderlands 2 hitting an enemy with slag doubled the amount of damage they'd take from a different elemental type. In Ultimate Vault Hunter Mode (basically new game plus-plus) enemies had such high regeneration and resistance that weapon-switching between a slag gun and something else was the only way to survive, and few were fans of it. New mechanics that don't force you into tedious gun-swapping (or worse, push one player into being "the slag guy") would be an improvement.
It sounds like radiation will work like corrosive damage did in the first Borderlands, lowering enemy resistances but also having its own damage-over-time effect to make it worthwhile on its own.
Borderlands 3 will release September 13 on the Epic Games Store, which will be the only place to buy it on PC until April 2020.