UNC (not that I want to copy everything that they do, but might as well start somewhere) actually has objective criteria for how they deal with this. According to here...
http://keepingitheel.com/basketball/retired-jerseys-2/
"Retired" Criteria
Oscar Robertson Trophy, formerly known as the United States Basketball Writers Association National Player of the Year
National Association of Basketball Coaches Player of the Year
Sporting News Player of the Year
John R. Wooden Award
Naismith College Player of the Year
"Honored" Criteria
MVP of a National Championship winning team
Member of a Gold Medal Winning Olympic Team
First- or second-team All America
ACC Player of the Year
NCAA Tournament MOP
Who from ISU's history would be in those categories -- substituting ACC for Big Eight and Big XII of course, if we were going to work off those standards as a starting point?
Ejim, Fizer, and Tinsley were all Big XII player of the year.
Georges Niang was 2nd team consensus All American
That's what I have off the top of my head.