Polls have shown that the maximum power-level players (a la EnWorld anyway) are generally comfortable for martials is Captain America/Black Panther levels. I associate that with the Hercules/Xena level as well and I can get totally on board with it for 14+ level PCs.
What I really
don't want is martials doing things "magically". Having them move so fast they seem like a blur is one thing--having them vanish or teleport or do anything obviously magical is not their purview IMO.
Now, instead of boosting martials, my appeal has always been to rein in casters, limiting spells to 5th or 6th levels. You can still do insane stuff with magic at those levels.
At any rate, since I seem to be in the minority on such things, I'll just express my POV and move on. Enjoy your games!