My personal favourite is James II. He has been incredibly mistreated by historians; even for the more progressively inclined historian, the Declaration of Indulgence was a tremendous triumph for freedom of conscience which would be unmatched for almost two hundred years in England. He is perfect for inclusion in Catholic hagiographical works; womanising heretic discovers the Faith, converts, loses everything for it, and dies in grace and at peace. His story is a terrible tragedy, but James himself was not too bitter about his situation. If it hadn't been for the duty he felt to his son and his rights, I have little doubt that he would have required to one of the monasteries which he frequented.
If the Stuart's had kept the throne or been restored, I could see Dom John Huddlestone being sponsored for sainthood. He sheltered King Charles II during his flight to France, and later confirmed him on his deathbed. Perhaps some day he could have come to be seen as the patron protector of the royal house.
I would have loved to see Margaret Pole canonised (someday, perhaps!). People have mentioned Katherine of Aragon (who deserves sainthood), but Margaret Pole suffered far more in defence of her faith, her queen, and her princess, and was undoubtedly a martyr.
I have always found Marie Julie Jahenny rather fascinating as a mystic. Perhaps a world where the Count of Chambord was restored to the throne might have seen her canonised as a prophet of the Second Restoration. Her visions were declared true by the Bishop of Nantes, and she's still venerated by traditionalists in France.
I was always a little surprised that Urban II was never canonised by a crusade-enthused later Pontiff. For that matter, I'm surprised that Godfrey of Bouillon wasn't.
Pius IX would have been canonised in the first half of the 20th century were it not for other Popes continually closing his case.
People have mentioned Blessed Karl; I have even more of a soft spot for the Empress Zita, who was by all accounts a holy mystic of a woman.
It would be interesting to see Prince Philip's mother Princess Andrew of Greece and Denmark (princess, nun, and righteous among the gentiles) canonised by the Greek Orthodox Church.
A few PODs could produce a world that has St Grigori the Healer... better known as Rasputin.
Another interesting little tidbit; Alexei Trupp and Catherine Schneider, two of the Romanov servants murdered at Ekaterinberg, were canonised by the ROCOR in 1981. This could pass without comment, but for the fact that Trupp was Catholic and Schneider was Lutheran; that's right, there is a Lutheran saint in the Orthodox Church!