You want a tiered system? there is one in place, normal/high/very high, have fun in unranked.

anything that isnt 100% win/loss dependant or a perfect system for quantifying performance WILL result in a forced meta and literally millions of ways of exploiting the system. It would be a clusterfuck, and it sure wouldnt be pretty.

You can be the best player on the team as a 0/0/0 treant protector or a 1/13/3 omniknight. How do you measure the efficiency of those heroes? I'm sure valve would like to know, because supporting on one of those 2 is a sure way to calibrate sub 3k even as a 5k+ player at the moment.

How do you evaluate a beastmasterplayer? Design algorithms based on how much vision you cover with the bird? Assign different levels of importance to different areas of the map? Gratz, it will be figured out and players will actively have to make the decision "do i make the right play and win, or do i gain mmr and lose". Guess what xxBoris19xX will choose. Make it depend on how many seconds your bird is the sole source of vision on the enemy team? also exploitable as hell. Discourages ward purchases.

How do you create an algorithm that manages to deal with both a game that is 50-50 at 20 minutes and a game that is 6-13 at 60?

MMR is good since it measures the only thing that is actually relevant. How many games you win. Once you start messing with it and evaluating HOW those games are won, you will be forced to play the game in a certain way in pubs.

And yes, it WILL be figured out. It always is. This game has ten million players, do you seriously think that there arent people who would spend years trying to find the ways to get to #1 on ladder by gaming the system? Anything else than win/loss encourages playing the system rather than playing the game.