The only witnesses allowed will be those with information material to the two charges, which are 1) Abuse of Power and 2) Obstruction of Congress.
Most persons whose testimony (if truthful) could hurt the Democrats have zero relevance to these charges. What, for example, would Hunter Biden know about either of those?
Just as the trial is not an open-ended investigation of Trump, it is also not a forum for investigation of the Deep State corruption and attempted coup.
“What, for example, would Hunter Biden know about either of those?”
He could show that Trump had a legitimate basis for wanting the Burisma corruption investigated.
“What, for example, would Hunter Biden know about either of those?”
Hunter might now KNOW any specific thing, but he can CONFIRM that POTUS had a legitimate reason for what he said in the call. Therefore, no abuse of power.
The biggest element of Trump’s defense against the “abuse of power” charge is that his dealings with Ukraine were a normal presidential function. Hunter Biden’s testimony would absolutely be relevant to this, since it would vindicate the President if it is determined that he was a crooked SOB who should have been investigated by the Ukrainian government.
An impeachment is first and foremost a trial. A trial has a discovery phase. The democrats have done all their discovery during the witch hunt investigation they held.
Why would you deny President Trump his right to discovery?
He can call ANYONE he wants to if they have even a small tangential connection to the trial. Which means EVERY piece of corruption on the left from Burisma to Jeff Epstein (who did not kill himself) to obama's many scandals (fast and furious etc) to schitt's and nadler's and pelosi's financial records, to pelosi's son working for Ukraine etc etc etc are ALL fair avenues of discovery.
All of these things go to prove that the Hunter Biden Ukraine corruption was just one small part of a RICO known as the democrat party. And as such it is the President's duty to root it out.