Opponent-free/Objective Metrics?

‪‪A lot of sports clearly depends on two teams interacting. Shots taken and allowed in soccer is dependent on the strengths of the teams interacting as well as their strategies. But which metrics can be used to determine the strength of a team or league (mostly) irrespective of opponent? ‬

‪An easy example is that free throw percentages in middle school are gonna be much lower than those in the pros, and even then it varies by team and player. The opponent has little-if-any to do with this number as well.

What other metrics might come to mind?