I always assumed Anna would be the party member shortly after the world started, and Elsa would become the party member after she gained control of her abilities- maybe even just for the final boss or something. I feel like with how often Anna and Elsa become more important than the other depending on the plot they could always be traded out for each other or be absent altogether, though, maybe only both being together for the last little piece.
Olaf has too much interesting potential for quirky animations to be shafted for Anna.