It's a really bad idea.
Firstly, Polaris's name derives from the term 'polarity', which is a property of magnetic fields giving them two opposite poles. It has nothing to do with chaos magic or probabilities.
Secondly, Scarlet Witch's name references her power to jinx things through her hex spheres that alter probability, making her look like a witch putting a curse, jinx or bad spell on something.
Thirdly, this franchise gets enough criticism for merging and combining some aspects of comic book characters and storylines for the big screen. Turning Polaris into a probability-altering 'witch' would cause more trouble than it was worth among the fanboys who, even as I write this, are somehow finding things to criticise even in the comic-accurate look of Psylocke.
What this franchise needs is to be more accurate to the comics and consistently make fantastic movies.