FormerlyHemlock
Hero
This reminds me somehow of the fact that folklore assigns various vulnerabilities and weakness to monsters which tend to get ignored in D&D 5E: vampires retain vestiges of these weaknesses (inability to cross running water, taking damage and temporarily losing regeneration while in sunlight) but it's not common to see things like e.g. mind flayers fleeing in terror from a rooster's crow, or nightwalkers being unable to cross a line made of salt. But you could!so that's a few starting ideas. What else do you think would make the high level world less insurmountable for martials?
You could also apply similar thinking to magical obstacles by requiring every obstacle including spells to have a mundane countermeasure or weakness. Perhaps Conjure Animals cannot harm anyone who is barefoot, and Wall of Force collapses if licked by a cow or stroked seven times with a raven feather.
This would incidentally boost "martials" but in a larger sense it really boosts Combat As War/puzzlegaming and mundanes of all stripes, including thieves and wizards who are using their "wise" brains and not their magical muscles. But certainly some warriors/martials could fit the Indiana Jones model of being brainy but not magical.