If I was making a high magic fantasy setting, instead of trying to worry about balancing casters against non casters I would just make everyone a caster and then build society around that. I consider that to be one of D&D's greatest flaws. You are making a tabletop ruleset - which is supposed to be played by people, which means it needs to be balanced - and then you are trying to tell me that someone who can manipulate time and bend reality is supposed to be balanced in comparison to someone who can't? I almost feel like its a bad joke. From my PoV, if you want to keep immersion, you either need to have a world where everyone is a caster and then a "fighter" is just a caster who uses melee spells, then you can make a balanced world, or you throw balance out of the window and tell people, "if you want to play a fighter, just know you will suck."
In the 2nd ed games I DM ed the fighters were the strongest class at the end. Especially the Paladins. At the point that the mage can stop time your fighter should have equally power items like a ring of 3 wishes and an artifact that has a number of spell like abilities.
Powerful magic items serve the same function that feats do but you aren't locked into them for the rest of your build. For me immersion comes from "adventurers are just rare birds"
I mean who really want to devote their life to crawling around in sewers hoping to find treasure in the guts of a carrion crawler?
