Nothing happens until after Senators question time

The Senate rules permit 16 hours of Senator questions. After that, the debate for witnesses will begin in earnest. It is likely that CJ Roberts will be drawn into a couple of rulings at that time, out of necessity, unlike CJ Rehnquist.

Perhaps he will rule that either side can call whichever witness they choose, relative to a majority vote of the Senate? Republicans should be able to call whomever they want, but it should be pointed out if the witness is for political reasons or is a first-hand witness to the charges in the impeachment?

Perhaps Democrats could call Mick Mulvaney and John Bolton?

As a surprise, perhaps the Democrats could ask that Don McGahn, from the Mueller Report, be called to testify? Republicans could not argue that he would have to be relative to the charges against Donald Trump.

There is a lot of maneuvering and strategizing left in this trial, unless Republicans choose to shut down the trial without witnesses.