While we are at suggesting Doctor titles to Pope Francis for Pope Saint John Paul II, I would like to put in my bid for Pope Emeritus Benedict.
He should go down as Doctor historicus for his major contributions in Revelation theology and Scriptural Historical Investigation within the Faith and for his central academic influence on universal Catholic doctrine for now over half a century and how timely his teaching is for our age (e.g. answering the dictatorship of relativism with the Truth which sets us free in history: God).
Or perhaps Doctor liturgicus for his indelible mark on the Liturgical revival: the Reform of the Reform!
N.B. It seems that the titles of Doctor are usually in the adjectival form and not in the genitive. In other words, the type of title refers to the man, his style, his personality, rather than the matter he taught. It's rather how he taught that is extolled. So John Paul would not be the doctor of mercy but rather the merciful doctor which I do not think particularly distinguishes him from any number of the other doctors.
We might call John Paul II the Doctor longus for the great length of his writings, bibliography, travels and pontificate. Or the Doctor peregrinus for his tireless globe trotting. Or even the Doctor passionalis for his artistic passion and great sufferings (e.g. the assassination attempt which brought him to the door of death, and the other time his spent weeks in the hospital again almost dying) and the long final illness to the point of spending himself before the eyes of the whole world.
Or, what about Doctor conjugius or Doctor matrimonius for his extensive development of the theology of marriage and family, so vital to so many marriages!
And, in that case Pope Benedict XVI should go down in history, again, perhaps even as one or other of the following...
Doctor brevis for his parsimonious teaching and final vow of retirement in silence.
Doctor academicus for his scholarly vocation.
Or Doctor monasticus for his undying preference for the hidden life, including his choice of name: Benedict.
Or how about, Doctor extraordinarius for the gift of the renewed universality of the Extraordinary Form!
We at Plinthos love titles of distinction for great men of distinction!