The question is actually much more tricky than it looks at first sight.
For native speakers, it is obvious it should be
Was ich sehe, bist du.
But it is kind of tricky to find out why.
What we know is: The subject of the sentence drives the predicate's verb form. The subject is normally a substantive in nominative, in sentences of the form "<etwas> ist <etwas>" you normally find two nominatives. One of them is the Gleichsetzungsnominativ, the second form of "etwas", the other one must be the subject of the sentence, which rules the verb. Now, which is which?
Der Löwe ist ein Raubtier.
So the subject could be either of Löwe or Raubtier (actually, it is indecisive, we can't really see without context what the subject could be)
Wikipedia tries to help with "try and replace the 'ist' with a verb that goes together with 'als'", like
Der Löwe entpuppt sich als Raubtier
Doesn't help much here, because it could just as well be
Das Raubtier entpuppt sich als Löwe
What we see in your example is that the first <etwas> is replaced by a relative clause ("Was ich sehe," we call that "freier Relativsatz", that term has the same status as a substantive and can also be subject or object in a sentence.
So let's do the same as above with your example sentence:
Was ich sah, entpuppte sich als Du.
Du entpupptest dich als was ich sah.
Doesn't help much as well, we can say it both ways, and both sound right. This form makes it clear where the subject is, and also what rules the verb form.
So, without knowing more context, we actually don't know which is right and which is wrong. Formally, there is no rule that says
Was ich sehe, ist du
should be wrong and
Was ich sehe, bist du
right. However, there is a tendency to pick the more concrete thing as the subject in such sentences, especially if the alternative is a "freier Relativsatz" - So 99% of native speakers would prefer
Was ich sehe, bist du.