A sequel is where the story is continued from the point where the previous game left off. Here we are seeing game names where a sequel is difficult to make, if not entirely possible. Case in point will be Max Payne and PoP. While Max Payne 2 tied of all the loose ends, making it difficult to pick up the story again, PoP cannot and shouldn't be made into a sequel. Its one of the few games that ended the triology in style. Continuing the story would be foolhardy and I am sure Ubisoft isin't going to do that. What they are doing is another PoP game with new character and new story line.
I am not saying that I am know-all. Even I myself have mentioned some games in this very thread that wouldn't come under the sequel category. There is no need to get all worked about it.