What is "merit" here?
Of course they will want children who can speak clearly and act, but there will be thousands of them. Plus, if they wanted the most proven child actors, they wouldn't run an open casting.
Which means they are looking for a type or a feel, for some kind of fit, which is the opposite of merit.
That said, I totally can imagine half white half Korean/Japanese Harry. Dark hair and a potential for a properly modern emo look for the later series.