If she can win all four grand slams - duh.
I think she needs to win another slam, maybe even three in order to keep a lead over others like Justine and Maria who play a full schedule compared to Serena's light [ only 10 or so ] tournaments. Remember Clijsters was # 1 with no slams while Serena held 3? [ 2003 American Summer ]. With Miami under her belt, winning Charleston, Berlin, or Rome will help her cause. It will depend on her outer-slam performances -- not her slams. She'll be #1 again but Year Ending? I don't know.