How would I go about a ranking system for players that play a game? Basically, looking at video games, players throughout the game make critical decisions that ultimately impact the end game result.

Is there a way or how would I go about a way to translate some of those factors (leveling up certain skills, purchasing certain items, etc.) into something like a curve that can be plotted on a graph?

This game that I would like to implement this is League of Legends.

Example: Player is Level 1 in the beginning. Gets a kill very early in the game (he gets gold because of the kill and it increases his "power curve"), and purchases attack damage (gives him more damage which also increases his "power curve". However, the player that he killed (Player 2), buys armor (counters attack damage). This slightly increases Player 2's own power curve, and reduces Player 1's power curve.

There's many factors I would like to take into account. These relative factors (example: BECAUSE Player 2 built armor, and I am mainly attack damage, it lowers my OWN power curve) seem the hardest to implement.

My question is this: Is there a certain way to approach this task? Are there similar theoretical concepts behind ranking systems that I should read up on? I've seen the ELO system, but it doesn't seem what I want since it simply takes into account wins and losses.