Don Fox on his knees was mentioned on the Wigan board I think and would be a good idea, one of the most iconic images of the Challenge Cup Final.
I hate him as much as the next man but could deny that this is one of the most enduring images too and would make an inspirational statue:
Beyond that I don't feel it should be a 'great player' from history like Edwards, Gregory, Long etc. because they just polarise opinion too greatly between rival fans. It should be a statue that all RL fans can be proud of - even if it is a generic, faceless team or player celebrating with the Cup.