Preferably someone who would convert to Protestantism after taking office.
Why in earth or heaven would someone who would remain in the Catholic Church long enough and gain the respect of his fellow cardinals to be able to become pope ever turn to Protestantism at that late date in their lives? Or is this to be the first step in creating a unified anti-Church and you're eagerly anticipating it as heralding the imminent return of the true Christ?
Possibly I could see a Pope take some steps towards a rapprochement with the Anglo-Lutheran branches of Protestantism, but certainly not towards the Reformed traditions.