While 5th edition does some good work to reduce the gap between Martials and spellcasters, I find the gap is there nonetheless. The issue is that magic takes on more and more of the non-combat aspect of the game. Climb and Jump are replaced with Fly, overland travel replaced with Teleport. Investigation replaced with divination.
Now I accept that magic is just extremely versatile, so its an inevitability at some point. However, while I respect that martials have to stay somewhat "mortal", I don't think they get enough anti-magic abilities. If the wizard can summon amazing magics, a fighters should be able to knock it down because they are "just that awesome". A 20th level wizard can alter the fabric of reality, and yet should be sweating bullets in the presence of a 20th level fighter....for no magic can take down this epitome of martial prowess. The wizard owns the non-combat stuff, but nothing is more feared than the highest level warriors in melee.
So with that in mind, I have made some subtle yet important tweaks to two of the core fighting classes. I may do it with the other martials as well, but I wanted to get people thoughts on this to start. Ultimately some of these are big power boosts especially at the highest levels, but I feel that when spellcasters are chucking 8th and 9th level spells around, this is not that big a boost at all.
Fighter:Indomitable. At 9th level, a fighter can choose to automatically pass a saving throw they just failed. They can do this 1/day. At 13th level, the fighter can do this 2/day. The spell has no effect, and the fighter takes no damage. If faced with a spell that doesn't have a saving throw, the fighter can use this ability to ignore the spell's effects for 1 round. At 17th level, the fighter can use this ability 1/round, in addition to his daily uses.
--Commentary: So the idea here is to give the fighter some truly epic defenses against magic. Forcecage, walls of force? Bah...child's play to the blade that can cut elder demons and ancient dragons. So periodically the fighter just doesn't have to deal with magic. He goes where he must, and no wizard can stop him. And then at the point where wizard's are chucking wishes, the fighter gains a general immunity to various effects. Wizard alter reality if you wish, but bring your magics upon the greatest fighters in the land, and you will be sorry.
Barbarian: Brutal Critical. At 9th level, a barbarian gains 1 extra dice when doing a critical hit. Further, 1/day they can turn any regular hit into a critical. At 13th level, the barbarian can do this 2/day. If the ability is used to target a spell that is destroyed with a disintegrate effect, the spell is destroyed. At 17th level, the barbarian can use this ability 1/round, in addition to their daily uses.
--Commentary: So the barbarian goes a more offensive style of anti-magic. First, they get a more consitent offense with their crits, but now they can use them to break through forcecages and certain other protections that are normally invincible. They aren't as generally magic defensive as the fighter, but they can actually destroy certain forms of magic with their attacks.
So with this, we have martial warriors that are significantly stronger at the higher levels than their current incarnations...but I think is more fitting to stand next to the spellcasters controlling weather, communing with the gods, and altering reality with a thought.