I took the time to read over the posts and get the general censous that people are unhappy with the Tier Systems and how poorly the reflect characters and are more hindering than they are helpful. Working off of that idea, why not simply ebolish the Tier System or any Ranking System for that matter?

It will be impossible to come up with an all encomposing system, and less is usually more anyways. I think it would be wise to simply point out the banned abilities. Time Manipulation, Self Healing, Battlefield Teleporting and anything else the powers that be decide.

Put a maximum power limit and let anything under that be made. Give some examples of what is possible, the speed at which things move, the destructive capability of some spells. It will limit some characters, and thats unavoidable regardless of which system you come up with.

Also, I think that the best way to temper character abilities is by having a team of people responsible for checking over characters and essentially giving it a thumbs up or down. 

You could also then tie it to a setting. Maybe the setting restrics, limits, or straight up removes abilities. Then people could play the super power characters they've had for years and have 'some reason' as to why they can no longer blow up planets.  Everyone would be brought to a relative equal playing field where writing skill will matter more than powers. All that is needed is a reason for people to want to fight. 

I'm rambling at this point...but thats my two cents.