First and foremost I am not making demands or saying this is even a great idea. This is just theory crafting, or a thought experiment. It was placed in general instead of the ideas and suggestions forum for this reason.
The fact of the heart of the question is players want to progress. When there is little for them to do at an “end game” situation people get bored. A new tier of hero would give them something to do.
Balance! Everyone has an opinion of xxx is to strong look at the top 100 it’s all xxx hero etc. By adding a 6* set of heroes you can wipe the game clean. Yes 5* will be viewed as weaker but everyone starts off with 4* then works up to 5* then if it was possible 6*.
Where else can we go? Hotm’s will continue to get stronger and stronger the gap now between tc 20 heroes and newly added heroes are starting to be in a league of thier own.