I've read that Hessian CO Rall received a note warning him of Washington's movements, but put it away without reading it. If he had read it, the Hessians could be formed up in the morning ready for the attack.
Additionally, Rall's engineers had drawn up plans for a redoubt north of town and other fortifications along the river. He chose not to fortify.
Also, Rall had requested that the British place a garrison at Maidenhead (Lawrenceville), about 5 miles from Trenton on the road to Princeton. So, you could have some British troops showing up at some point (2 to 2-1/2 hours after the shooting starts, or earlier if warned.
And there is always the "forgotten by history" battery of artillery at Trenton (or was it a squadron of cavalry?).
Perhaps a company or two of Jaeger (again not remembered by history) went out on patrol despite the weather and engaged the Rebels before they reached town.