I'm just an amateur in this area, but here's a suggestion...
The war ends months later; the Rebels still win, but they don't win as much. The Old Northwest (and perhaps even a chunk of upstate New York) could remain British Territory.
Or is that totally wrong?
Your general idea is right, but the land concessions are wrong. Up until this point, the British were in a strong position in the south, not the north. They still held New York and perhaps could expect to gain a hinterland for it in a peace, but really they had abandoned the north years ago. Instead, they held a firm (relatively) control over Georgia and South Carolina, they were in nominal control of North Carolina, and were pressing north in the hopes of clearing a path all the way along the coast. The loss of the only remaining major British army then meant they had no forces with which to hold their ground from a concerted attack, save for militia and garrison units, which weren't going to hold off the Continental Army. If Cornwallis' army stays intact and isn't routed, butchered or captured, then the British have every chance of holding what they've taken - they were starting to adapt to the war, and had a much better chance of holding the southern colonies alone as it didn't require nearly the commitment of holding the whole 13 colonies, and saving another calamitous defeat, on the defensive he had every chance of fighting back any Continental Army incursion. Should Cornwallis have survived, he likely either would have pressed ahead to his objective doing serious damage if not ultimately successful, or headed back to defensive positions; the likely outcome is a peace a little sooner with the UK retaining everything south of North Carolina (including NC) plus as mentioned New York City. The British would likely have been in a good position to negotiate for their version of the Maine border, too. Otherwise, I find it unlikely to see the Americans losing any more of the north.