the various ways in which martial/mortal/natural/"normal" progatonists can be made viable within a roughly D&D-ish mechanical framework
Well, one of the more obvious solutions is actually pretty much what Batman does: Make their own gear.
Imagine if a D&D fighter did not have to depend on DM handouts to get their magic items. They could craft their
Flametounge or their
Holy Avenger themselves. "Craft" is an ambiguous term -- they could dunk their armor in the blood of a red dragon and make it
Armor of Fire Resistence. The don't "cast magic" per se, but the weapons and equipment they wield become enhanced.
From an out-of-character perspective, they could simply select powers. "Oh, I'm level 3, I'm going to gain the
Armor of Frost ability that lets me freeze enemies who hit me!" Next adventure, they have it.
From an in-character perspective, they could stumble upon their item ("Guess what I found under the chicken coop!"), they could craft their item ("Using Undying Ice from the Frostfell, I craft a suit of armor!"), they could
pay someone else to craft it ("Dear Dwarves of Glacier Mountain..."), they could be rewarded with the item ("Huh, guess the Queen of Winter liked the way I killed that dragon..."), they could have it delivered by an international support network ("Thank you for this most recent gadget, Q!"), or they could even discover the way to make that happen from simple research in a fantasy world ("Oh, I guess if I bend the metal
here, and add this bit of dragon's tooth
there, and leave it out in a snowbank overnight....").
Same way Spellcasters learn new spells, warriors gain new items.
Armor. Weapons. Boots. Rings. Rogues are no longer outclassed by a wizard with
invisibility, because they get to choose their very own
ring of invisibility at about the same level.
This
also frees up the DM to award more random treasure. Rather than useful, narrow, practical things, the DM can play with magic items with a heavy cost, a weird theme, or a slightly divergent theme. More wondrous items, less +1 things. The +1 things are right there for adventurers to make/find/whatever themselves, automagically as they level up.
So a hypothetical D&D Batman goes from a
Batarang +1 and a
Cloak of the Bat at first level, all the way up to
Kryptonite Bullets and a
Helm of True Seeing (or whatever).
This also meshes with the myths, and lets characters customize their gear to their hero's theme.