For Cole and other slower posters, proffered testimony has to be relevant to the allegations/articles against Trump.
Assume Trump was in a close primary fight with a GOP challenger. Let’s call her Nikki.
Assume Trump withheld military aid to Ukraine because Trump asserted that the Nikki was a silent financial partner in a Ukraine oil giant, with strong ties to Iran and North Korea. Assume Trump refused to release the military aid and refused to give Ukraine’s new President any public recognition, unless the new Ukraine President announced a public investigation against Nikki.
Assume Trump is impeached.
At his Senate trial, he should be able to present evidence showing why he believed in real time why Nikki was in bed with the shady oil giant with ties to Iran and North Korea.
But, he couldn’t call Nikki as a witness, as any new information developed during questioning Nikki at the Senate trial was not known by Trump at the time he pressured Ukraine and withheld the aid.