This is actually something I've thought about from time to time.
Possible POD's:
-Cresap's War expands, and Virginia and Pennsylvania split Maryland with royal consent. Due to the Pennsylvania colony increasing to a much larger area than the original grant, George II incorporates Delaware officially within PA borders, (as PA administrated the colony until independence anyway.) With two small states off the board, the Constitutional Convention goes in favor of the large states for too long to affect a resolution. Virginia, Pennsylvania, and New York unite, and eventually annex New Jersey.
-Colonial operations in Canada go more favorably, and by the end of the war, the US is ceded everything south of the St. Lawrence, plus PEI. This, in turn, also twists the Convention, with the added issue of administering the new territory.