NO more witnesses. The information gathering is what the House was supposed to have done before submitting impeachment to the Senate. The Senate simply looks at what the house presented. The Senate is not responsible to conduct yet another investigation.
If they allow any witnesses, they should start by cross-examining those who have gone before this. And then they should strike each piece of testimony as hearsay or opinion. When there are no statements left, dismiss the articles for lack of evidence (or lack of crime).