What's your favorite off-class build? (a skill that isn't in the skill tree of that class)

Zeal barb?
Enchanting Zon?
Some type of godly fools build?

I want to try something new and looking for ideas!

Portal barb has really been hitting the spot

Cold necro
Lightning druit
Revive palidan

demon machine fire builds if they count on Pala or Sorc. Even Warlock.

ww sin I like but is bad at pvm
trangs fire necro even worse but is fun floating around

Zeal sork eZ

I once read about a melee necro that used brand to pew pew bone spears while in bear form. I didn’t try it but it was a comical read. However, not quite fitting your post since it’s one of his skills.