I just give an example: you and a friend together to eat lunch, the waiter will ask you“您要什么？”,he just want to know an answer; then you maybe ask your friend“你想要什么？”,that mean you want to know what your friend want to eat, that must be the food he is interest. And your friend do not hear what you say, he looks absent-minded, then you ask “你想什么呢？” I am not sure "what do you want" in which mood, I think "你要什么“”你想要什么“ both OK under different condition.

你 要 什么 = 你 想要 什么 stronger than 你 想 什么 by degree of intention

when you ask someone "what do you want to do", use 你想要干嘛/干什么, "what do you want", use 你想要什么, we don't use 你想什么, it equals"what are you thinking".

As a Chinese，in Chinese language habits ，the answer is “你想要什么”，