There might be some kind of technical rule that says you are not supposed to say "will want" in a formal situation or in writing, but I can assure you that, at least in the United States, people say it all the time.
E.g.: "I'll want that money by tomorrow or I'll break your legs." "I'm sure that he will want to learn Portuguese before he gets to Brazil." "She won't know what she'll want after she gets a PhD in existential philosophy."