I can’t really see Blizzard wanting to attack anyone for the use of the word “Cataclysm”. It’s just a word to mean “a hypothetical doomsday diaster or any kind of large scale disaster”: if one of those happened, are Blizzard really going to go and complain when someone uses the word?
There’s also a Batman comic that uses the word which, to my knowledge, hasn’t been taken down or forced to change it’s name. Considering HW: Cataclysm and the Batman comic precede the Blizzard use, I very much doubt there wouldn’t be much use in using “Cataclysm” in any remaster or mod of HW: Cataclysm itself.
Of course, Gearbox would obviously want to get permission to use the word from Blizzard themselves before they develop any official DLC or game for HW: Cataclysm, but I doubt they’re going to say no, considering that the WOW expansion has no relation (or even relevance) to HW: Cataclysm.
I am not a lawyer, however. Just stating my opinion in this, as it’s interesting to me.