What I mean above is that 你要去哪里can be a way to ask someone where he or she wants to go (by him or herself) or where he or she wants to go with you. "Where do you want to go?" usually is a way of asking someone where they want to go together with you. "Where are you going to go?" clearly means that you are asking the person where he or she will go (without you). "Where do you want to go?" could be used in the same way. OK. I think you are probably more confused now! It basically means "Where do you want to go?" but not "Where are you going?" that would be closer to 你去哪里？or 你要去哪里玩？

where are you going?

where are you going ? or where will you go?