I wanted to write, "What would you do if you were invisible? " but I guessed that DL would not have accepted it. Would I have been wrong?
"What will you do if you are invisible" is not a construction that works in English.
It's grammatically allowed, but sounds really odd since we use "would/were" to talk about hypotheticals while "will" is reserved for things we expect to actually happen.
If "invisible" is taken figuratively, in the sense of not being noticed or recognized, the sentence could possibly occur in English.
It still doesn't work in this sense, because it is hypothetical and needs would.