Hello Southern Chicken,
If that could be arranged in a way which would protect that individual's identity then perhaps that might be agreeable. It should be understandable that many wish ill will towards that individual, so revealing their identity would place them in great mortal danger. Also, most of the evidence which was identified by the WB has been supported, repeated, corroborated by others. The WB is not needed to make the case against the President. Also, it is unlikely that the WB will offer any information in defense of the President.
The President is free to offer any key witnesses with direct information about the case which may dispute any of the charges made. Let us hear from Giuliani or Mulvaney, for instance. Trump could testify himself, as Clinton did in his own impeachment. Is Trump afraid to come forward and be sworn in? That does not speak highly of innocence. That leads us to suspect he is guilty. Why not come forward and tell his side, under oath? He seems to be extremely willing to explain his reasoning via twitter as long as he is not sworn in... Do Republicans only wish to hear from the President if he is not sworn to tell the truth?
A WB in any case must be protected from retribution or else no one with inside knowledge will ever report any wrong doing ever again. That is why we have WB protections.