I’m going to be a bit of a contrarian on this one, at least potentially. If DJT Jr appears, it opens the door to examine how the meeting with Natalia Veselnitskaya came about in the first place. We know that Loretta Lynch issued a special exception to her visa allowing the Russian lawyer to stay in the US to attend the meeting. This strongly suggests a setup by the Obama Administration to try to claim collusion. If DJT JR chooses to testify, this whole line of exploration is opened up.
I can’t remember all the details I read, nor do I read everything. Hopefully others know/remember the details. But I think the hows and whos the of the Natalia Veselnitskaya meeting are already known. Pretty sure I’ve read about that more than once. I’m sorry my feeble wits can’t remember details, though.