A rebuttal witness? The Grand Jury questioning of witnesses is controlled by the Prosecutors, it is not an adversarial process.
The only explanation would be if Trump is going to testify, and the DA wants to have Cohen on hand to tell more lies in order to frame Trump. If not Trump it could be someone friendly to Trump that Cohen will rebut, but why would the DA call such a person? The prosecution is under no obligation to present both sides of the story, they just have to present enough evidence to get an indictment.
1. The alleged NYC charges are predicated upon the presumption that Donald Trump made an "illegal" hush payment to Stormy Daniels.
2. The only way the allegation rises to the level of a felony would be if the payment was made as part of a larger criminal undertaking.
3. The alleged "larger criminal undertaking" was a violation of Federal election law.
4. Trump has not been convicted of any crime related to any Federal election law. He hasn't even been charged with any such crime. In fact, he probably can't be charged with any such crime (due to the statute of limitations related to said crime) even if he came out tomorrow and admitted that he committed such a crime.
5. So the NYC mutant prosecutor is left with this bizarre case where he's alleging a local/state crime related to a Federal campaign finance violation that doesn't exist.
6. He probably feels like he's got to call some jackass witness to fabricate some kind of legal house of cards where non-existent Federal crimes count as "crimes" in his state case.