If I tell you that I have just seen a terrific movie, I might then add 'you will want to see it" - meaning that I am encouraging you to see it, because I think you will enjoy it too.
"You may do' is used to give permission. You may do whatever you want once the exam is over (although many, many English speakers use 'can' here instead of may).
If I tell you that you will do something this can either be me telling you that you must do something - "you WILL pay back the money, or else", or just stating something that I know will happen in the future - for example, 'when you land in Sydney, you will notice it is much warmer than London".