I'm going to have a different point of view than Andres F, who I don't totally disgree with, though. My answer will be directed to the in-question question, not to the in-title question (Why doesn't he just take Winterfell?)
In Westeros, right of conquest is as sacred as heirship. Robert Baratheon is a proof of that statement. He conquered the Iron Throne. The fact that he was related to the Targaryen (by some old marriage a few generations ago) was merely an excuse. He conquered it by killing the Targaryens. Some people call him the Usurper, but he is the rightful king. This is why Daenerys can't just show up saying "Hi, I am the rightful Queen, please let me the throne". The king would just kill her... because she would be a traitor.
So... Tywin Lannister could just take Winterfell, but he would have to fight for it. A marriage with Sansa Stark make him get Winterfell without fighting a single battle. And Tywin Lannister is not a fool. He knows that it will take every worker available to sustain the coming winter, and this means not losing those workers in battle against the northeners.
He wants to make a Lannister heir of Sansa Stark's hypothetical son because it is the easiest way to get Winterfell.