Ye olde fighters vs wizards debate to some extent. D&D is a fantasy world but the fighter has always been a somewhat mundane class. He has more or less been restricted to what someone in the real world can do with some level of abstraction such as being able to survive being breathed on by a Dragon. Or in Spelljammer being able to survive reentry (20d6) and then 20d6 falling damage and being able to leap up and keep fighting.
Fantasy fighters and heroic fighters often had magic items/artifacts (Excalibur, Stormbringer), gifts from the gods (Perseus) or were part divine themselves (Hercules). In fantasy literature worlds are often low magic as well (Tolkein, Game of Thrones) where spell casters are very rare or magic is more limited than D&D magic. A simple levitate spell makes the walls of Constantinople somewhat obsolete but Mr Fighter can ride out on a warhorse with his lance-yay. 2nd ed low magic worlds often nerfed wizard in some way usually by increasing casting time or limiting their access to spells (or both). The high point of being a D&D fighter probably came with 2nd ed or 4th ed the low point was 3rd ed relative in power to the other classes. Perfect balance is probably unattainable at a price the D&D fanbase is willing to pay as 4th ed found out regardless of ones opinion of 4th ed they at least tried I suppose and one can respect that.
Feats are apparently coming back and one can sacrifice them to gain higher ability scores. This is somewhat interesting at least and may appeal to fans of older editions of D&D. With capped ability scores on cannot abuse this like 3rd and 4th ed but it is nice and simple and having 20 strength/con/dex does sound appealing mechanically while being somewhat mundane.
Anyway feats are back with an opt out clause for those who do not like them. We do not really know how many the classes will get access to, nor how they are going to be implemented. I suspect we will end up with a large amount of them a'la 3rd and 4th ed. Some ideas to make fighters better although I would not expect to see many in core rules perhaps and some should probably be kept out of the core.
1. Magic items. Maybe detested but this was the way things worked pre 3rd ed and it required DM fiat. Gadgets could also be used- Batman, Bond and Ironman could be fighters or multiclassed fighters.
2. More mundane options that are powerful. Reroll abilities (advantage), extra hit points, defenses, ability to sever limbs/heads and puncture lungs like Jedi in SWSE are all mundane abilities that can be represented by in game mechanics. Higher ability scores have already been mentioned.
3. Give them super natural abilities. 3.5 Dragonmarks and bloodline feats, 4th ed power feats and multiclass feats, SWSE force training etc. These let fighters gain supernatural abilities but not become spell casters. Maybe your dad was Zeus or Grazzt and on a fantasy world why not?
4. Super heroic. Similar to 3 I suppose but with more or a comic book or wuxia focus. Fighters jumping to the moon, balancing on bamboo etc. In D&D terms maybe Superman is a fighter.
Just some thoughts and I would not add some of them to the core rules at all.
Fear is the Mind Killer