Kim Gu is the best candidate. Assuming that Korea wouldn't be divided and that Kim-Il-Sung would be neutralised in a way or another, Rhee's anticommunist platform wouldn't be so appealing to Koreans nationalists, and Rhee himself, quite controversial since the old days of the Korean provisional Govt in exile, would probably be far less popular. Prince Gu could be then crowned Emperor with some Regency system. Still, Gu and Kim would have a hard time to make Koreans forget the failure of the dynasty to protect Korea's independance.
Incidently, a Korean drama series called "Goong" (or Princess Hours in English), that has been extremely popular in ROK and in Asia as well, is based on the premise that a Korean monarchy has been set back on the throne after 1945, the story revolving around the princes' and princesses' antics and love stories while the current Emperor is dying.