What is the significance of Twelfth Night's alternative title, What You Will?
In my opinion, this play has this particular alternative title because it is not a very serious play. The phrase "what you will" means something like "whatever you want." It seems to me to indicate that Shakespeare does not take this play very seriously.
As I understand it, this was a play meant to be put on at the climax of the Christmas festival season. This would mean that it was supposed to be a very light comedy that everyone would enjoy -- something sort of mindless.
I think that the alternative title reflects that.