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4th edition supported levels up to 30 in the core rulebook, while previous editions supported levels up to 20. What level would you like to see the 5e core rulebook go up to?
I run E6 because I think 3rd and 4th editions both get too powerful and wacky at higher levels. If 5e flattens the power curve, I would not be averse to 20 levels, but I will still probably keep my own campaign world to 10-12 levels.
Somewhere between level 10 and 14 is where it should stop.
Then, have different modules depending on where the group wants to go.
Some kind of mythic module for those who like amazingly powerful protagonists and one module that is more about characters gaining reach in the world (domain management and stuff like this).
I would lower the level limit back down to 20 and fold epic rules into that level range. In other words, compress the 30 levels of 4E into a smaller level range. As a whole, 4E classes, monsters and magic items felt a little stretched thin across 30 levels. At the same time, I don't want epic level stuff kicked out into a weird post-game format like in 3E.