Of course, using a sword is so simple that anyone can pick one up and use it competently within seconds. Why all those warrior cultures wasted years of time on training when it's that simple is beyond understanding. It breaks all sense of verisimilitude that there's any training required to pick locks like a thief, or identify trail signs like a ranger.
No actually, that's my whole point. The wizard has studied for years to cast his spells. The fighter has studied for years to build his strength, speed, stamina and skill. The ranger has spent years learning herb-lore and tracking skill and sharpening his skill with a bow.
None of them have had time to learn what the other guy knows. So no, the high level fighter does not throw fire balls or fly. Likewise the high level wizard cannot fend off 6 angry orcs with axes. And neither of them can track a pack of nomads across desert hardpan or through a swamp.
If magic is accessable enough that you can pick it up while doing something else then there should not be any such thing as a mundane class. And indeed, historically there isn't. While magic does not have any great functional track record in our world a Knight still sat in holy vigil over his arms before being knighted, seeking supernatural favor in battle. A Samurai would perfrom the ritual of the bow before shooting and would not loot fallen enemies to obey Shinto strictures against touching the dead. Warriors everywhere and everywhen carry talismans and lucky charms. "There are no athiests in foxholes."
But D&D traditionally has a hard line between magic using and mundane classes. This is based on the literary conventions of the sword and sorcery genre where usually wizards are bad guys who got their power by making dark pacts or delving into unhealthy and forbidden lore. In these books magic is not easy or convenient or widely practiced.
And if we are to retain that traditional divide between magic-folk and muggles, then the muggles don't get to do magic. Ask your wizard buddy for a buff spell or a magic item? Sure. Leap 40 feet into the air or throw a guy across a room? Not so much.
Yes this kind of stuff happens in wuxia. No, it's not mundane there. Monks are
monks. They use chi, harness chakra, achieve daoist enlightenment and
write magical scrolls! They are the perfect model for how you have a magic using martial character because that's what they are within their own mythology.
Now could you explain monks, wizards and knights all existing in the same world? Maybe. If Iron is as anti-magical as legend would have it, that could explain everything. Knights don't use chakra and chi because giving it up is the price they pay for walking around in suits of anti-magic material.
At any rate my point is not that fighter have to suck, it's that you either have to limit them to physics, or quit pretending that they are mundane characters. I'm actually perfectly fine with leaving the whole adventureing muggle concept behind, but it's not what D&D had traditionally been. It's far closer to Earthdawn.
All I'm asking is that whatever they can do
explain it! Maybe Kord grants blessings to anyone who has slain enough foes with a sword to gain favor in his eyes, and that's why high level fighters can do the impossible. I could make this stuff up all day long.
But if you have a magicless character he should not be able to do anything you or I could not do.
If a character does have magic, then I want it to
be magic damnit! It should be weird and wonderfull and maybe dangerous. It should be subtle and spectacular by turns. If it's rare it should be hard, and if it's easy it should be everywhere.
It really is not
that hard to make rules which portray an internally sensible world.