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I would think that Trump's interview is the last one to complete their investigation unless Trump messes up. It may be just a formality or one last opportunity to justify their whole reason for continuing this charade for this long.
More likely a bluff. Yes, Bill Clinton was compelled to tesify in the Paula Jones case but that was a civil suit. (Bill still committed perjury but that isn't relevant to this situation.)
Mueller's probe is criminal, not civil. He would need to show a judge why he needs Trump's oral testimony, i.e. that an underling on the Trump campaign or during the transition has somehow implicated him on something.
That having been said, most plausible avenue to me are the Comey allegations of obstruction in the Flynn matter. A he said/he said situation since they were alone. Even then, Mueller would need to convince a federal judge which facts could support even bringing obstruction of justice charges against a sitting President when the investigation continued and Flynn pled guilty. High hurdle.
Of course the state of the current federal judiciary is so bad, the Supreme Court has agreed to take unusual measures to short circuit both the 9th and 4th circuit Courts of Appeal on the travel ban case. When a majority of the Supreme Court has lost faith in the competency of the federal judiciary, it's that bad.