The design is a good thought but once further explored a very big problem comes up. Tanks need to be able to do a least one of two things. Those things are having kill pressure and providing constant CC. By kill pressure, I am referring to the ability that a tank needs to be able to kill a target if the tank champion is able to reach them. I am not suggesting a one shot, but that the damage of the tank will outsustain/kite the target. For my justification of for why this needs to occur, let's take a second and look at what happens if this isn't the case. An extremely tanky champion gets to a carry but isn't able to damage them or CC constantly. The carry can, literally, just walk by the tank and focus a more vital member of the enemy team. This means that the tank will have very limited impact on the game as even if they are near not able to be killed, they will be ignored in team fights. Now, obviously this is an exaggeration but the point is clear. Being extremely tank alone isn't that useful. You need the damage or CC to actually threaten the enemy champs. As for people who would suggest that you can just reduce damage and increase CC, too much CC can lead to certain classes becoming very hard to play (eg. assasins) and lanes to become too gankable. The idea you have is decent, but one must look at how stats interact with the desired role of a champ and what the role of the champ should even be.