In this context, yes, I would use the article: some people would like to go back to (return to) the franc. I would not capitalize it. I would also be inclined to insert "French" in the sentence: either "French people" or the "French franc." On the other hand, that may unnecessary if it is already obvious from the rest of your text.
When speaking of currency as a sort of entity, yes, it generally takes an article. (I admit I would not be surprised to see someone prove me wrong...)