Well there is no set scenario because this is all a hypothetical potential way of going about a contract. So potentially to hard.
However, you would start with KPIs for things in the players control such as conditioning (weight and/or skin fold requirements, beep/yoyo targets) and training (attendance, on-time attendance percentage targets).
As for on-field targets the traditional method is games (and grade) played. I don't know enough about Will Craig to relate it to him specifically but by say age 19 any back should be nsw cup level (perhaps a little older for forwards). So it would be play 12 games or more of cup to trigger the next year. You could change it to something like percentage of games available you are selected in the top expected grade for example 70% of the games available he should be picked in ressies. It is common for fringe players to have extension clauses put in for games played e.g. Klemmer had one.
US sports often have stats based bonuses. I'm not really big on stats based clauses unless it was for something specifically being worked on by a player but that could also be an option.