THERE'S NO GOOD DEAL TO BE MADE

This seems naive:

... behind closed doors, a small group of Democratic senators and aides has begun ... sounding out colleagues on whether to back a witness deal that could lead to testimony from former national security adviser John Bolton or other administration officials with possible firsthand knowledge of the Ukraine controversy, according to multiple Democratic officials who spoke on the condition of anonymity to describe the private discussions.

These Democrats said they believe having Hunter — or possibly Joe Biden — testify could backfire on Trump and the GOP, giving Biden and the party a platform to strike back and paint Republicans and the White House as obsessed with trying to damage one of Trump’s 2020 presidential rivals.

Maybe a one-on-one deal -- uncensored public testimony from John Bolton for testimony from Joe Biden -- would work to Democrats' advantage. But that's why such a deal will never happen. Republicans will demand Hunter Biden, Adam Schiff, the whistleblower -- and/or some of the people mentioned in this Wall Street Journal editorial:

There’s also former Obama energy czar Amos Hochstein, who raised concerns with Joe Biden and his aides about Hunter’s Ukrainian ties. And let’s hear from Chris Heinz, former secretary of state John Kerry’s stepson, who broke business ties with Hunter because his Burisma work was “unacceptable.”

Any attempt by Democrats to put chips on the table is likely to be met by Republicans upping the ante, with the result that, if a multi-witness deal goes through, either the Bidens or the impeachment process will be on trial, or possibly both. It's naive to believe that Susan Collins et al. truly want witnesses if the witness deal puts the president at risk (which a John Bolton-Joe Biden swap would, because Biden has a solid story -- he truly was sent to Ukraine to pressure the country to fire a prosecutor universally regarded as corrupt -- while Bolton's testimony really might not exonerate the president). Collins wants to be seen as fair, but not at the expense of her party or her president. Senate Republicans will declare any proposed deal that doesn't create an anti-Biden or anti-House Democrats distraction to be unfair; they'll vote no, with Collins and Mitt Romney on board.

So, sure, give it a try, but don't expect to get these folks to agree to a reasonable deal. It won't happen.