I like how North Carolina deals with Jersey retirements. They've honoured 51 players and retired 7 jersey numbers, with the following requirements for honoured players:
North Carolina requires that players must be recognized for excellence for their performance during the regular season, in post-season play, or in the Olympics. Specifically, a player's jersey qualifies for honoring if he receives one or more of the following five awards: first- or second-team All-America on one of the major All-American teams that qualify a player for a consensus All-American designation, an ACC Player of the Year, the Most Valuable Player of a National Championship-winning team (as voted by coaches and teammates), the NCAA Basketball Tournament Most Outstanding Player of a Final Four team, or a gold medal in basketball at the Olympics.
For full retirement a player must win one or more of the following six national player of the year awards: Associated Press, National Association of Basketball Coaches, Sporting News, John R. Wooden Award, Oscar Robertson Trophy, or the Naismith College Player of the Year.
Getting that national player of the year in college would be harder than an MVP in NBL I believe, given the number of teams and players. But could nearly give a retired jersey for MVP in that scenario meaning Randle would qualify which would be controversial for some. But IMO Maher and Davis are the only true past legends of the club that deserve their numbers retired, unless you count historical players before sixers like Al, who had his best years before 6ers. But having guys like Ninnes, whilst a good player, was more a celebrated role player, shouldn't qualify for jersey retirement.
So my requirements for honoured player is First Team All NBL, DPOY, MVP, Finals MVP, 200/300 games(?) etc. Then as a retired number to have achieved outstanding performance over a set period of time, eg an MVP like Randle or Finals MVP like Brooks would have to have the 200 game minimum to qualify. And a player like Johnson having won awards including 5 club MVPs would qualify for it when he retires.