The question is just like a conversation between two persons one of the is making a "Kostümfest" and asks his friend about his "Kostüm" .How could he ask him using "als" in the beginning or is there any other way to ask about that.
Notice that the respond to the question is : Ich komme als Superman .
Thanks in Advance.
Als was kommst du?
Is a quite common form of this question. A longer form could be:
Wie / als was kommst du verkleidet / kostümiert?
Du kommst als was?
is less common, could be used just for variety, if asking several questions / several people. If you put the stress on "was", it can be the response if s/he tells you s/he comes in some surprising / outrageous costume.
As already posted by Bobby J, "Als was kommst du" is totally OK.
But if you want avoid "als was", you can use "In welcher Verkleidung kommst du?" or "In welchem/was für einem Kostüm kommst du?".
