In a real world trial, whether for civil or criminal, each side would call various witnesses and provide documents to support their argument. The witnesses called would have to be relevant to the issue in the trial, otherwise, they are just red herrings and add nothing.
Trump's impeachment trial is not about a crime he committed, it is about his conduct as POTUS, in this case obstruction of justice from Mueller to Ukraine, POTUS has obstructed even the impeachment (ignoring subpoenas, threatening others not to cooperate, refusing to turn over any documents sought, etc.). With Ukraine, the GAO stated that Trump's withholding aid and the insidious back channels to do so, was unlawful but not a criminal crime, more of a civil offense since Congress had passed it long ago and Trump halted it to gain a favor that ONLY he would benefit from. That is not in the interest of the USA.
Republicans want Hunter Biden because they want to smear and throw red herrings about his ties to Ukraine. Hunter Biden was hired by a private Ukrainian company, Burisma, making $50,000 a month. Whether he has experience or not, Burisma has a right to hire anyone who they think would benefit their company. Hunter was hired during the Obama presidency and his dad, Joe Biden was VP. While the optics are bad, there has never been any evidence from other investigations that Hunter did anything wrong except make a lot of money because he was the son of the US Vice President. This is no different that Ivanka Trump's benefiting from her father being the current US president with China. She has gotten over 20 copyrights\trademarks from the Chinese. Hmm, I wonder why? The Republicans hate to even talk about this.
Hunter Biden knows nothing about Trump's obstruction charges since they occurred long after Hunter was at Burisma. He knows nothing about Trump's withholding the aid to Ukraine in order to get a favor from the president (to launch an investigation into Joe Biden) as he was a private citizen living in LA. What could he possibly know? Did he and his dad talk about it years back, maybe, but so what? Hunter Biden knows nothing about the back channel Trump created.
On the other hand, John Bolton, is a key and relevant witness to nearly everything about the Ukrainian aid. In fact, he wanted nothing to so with it because he knew it was not legal. Trump refused to allow him to testify for months fearing what he would say. Now, Bolton states will testify regardless of what Trump wants if the Senate subpoenas him.
Leslie McCowen from Cape Cod, USA on January 19, 2020:
I heard Trump will claim executive privilege on all of them. So, dems can then take it to Court and it plays out for years to come.
Or, he can shoot somebody on 5th Avenue, they will blame Biden, throw him in jail, and Trump can go on to his lifetime, god-given post as king of the world.
God forbid he do as Clinton did, submit himself to the mistakes he made, apologize and try and make amends......
That would be the patriotic thing to do, so.....that will never happen.
Either we play it out in court, or get used to a unitary executive. I/e not America.