The principle ground I would rely upon is the denial of due process by not informing Donald Trump of the underlying federal crime he was alleged to have committed—the predicate crime that NY law requires to prove the NY crime.
That is a kangaroo court: to try someone for a crime that they are not informed of is as serious a breach of due process rights as we can possibly imagine. See, e.g., the Stalin show trials—Darkness at Noon.
The timing of the trial and the blatant denial of fundamental rights can only be explained by an effort to interfere with the fair conduct of a presidential election, thereby, in effect, denying the American people the right to a Republican Government.
Plus Merchan allowed the prosecution to testify that the mysterious “underlying crime” was a valid charge.