I want Gwen to be in movie one, with Mary-Jane as a passing flirtation.
In movie two, I want Peter and Gwen to be all out in love and have the film culminate with her death, but have Mary-Jane and Peter (and Gwen, Harry, etc) all become much closer friends.
By the end of movie three, Peter and Mary-Jane are now together, and the death of Gwen had something to do with this....maybe even end the trilogy on the same kind of note as Spider-man: Blue where it is understood that Peter had TWO loves of his life. That Gwen was the first and he still loves and misses her, but he's moved on and now loved MJ and she is fine with knowing this, because she loves and misses her friend, too.
-R