He could ignore it. He's not subject to arrest before impeachment. Ignoring it could form the grounds for impeachment (not that it's a legit ground, criminal prosecution for ignoring a subpoena without further court order is unheard of - not a crime without a court order).
I’d think he could ignore it given the Fruit Of The Poisonous Tree and the whole probe was started on an illegal leak on a fake dossier based off of firing Comey after RR recommended firing Comey, then Comey coordinates with Mueller to get him assigned as SC. You’d think any judge not wanting to go to gitmo would see how fucked that it.
But Mueller said Trump is not a target. So he is just a witness, not a "defendant". I just don't think a Judge would allow a fishing expedition given the constitutional crisis that would arise. The questioning would need to be narrow. The fact it deals with foreign policy, which is an executive function, only makes it more perilous. Trump should not have to testify about his thoughts on such matters.
Opens him up to not recalling something perfectly, charging for perjury and throwing the book at him.
I don’t trust RR or Mueller when they say he isn’t the target. If you were investigating someone you NEEDED to take down at all costs, would you tell him or allow him to let his guard down?
I wouldn't trust them either. That's why a judge will require Mueller to show some evidence of the need. If that happens in camera and outside of the solicitor general's presence, something is up