I've heard it said that Fighters aren't underpowered, boasting the best battle endurance and 3+ attacks. But in the 5-10 levels, other classes have fighting styles (or rage) and extra attack, with other features that outshine the fighter's. 

Specifically it's the Paladin v Fighter comparison that disappoints me. Smites last longer than action surge, lay on hands is more powerful, reliable and flexible than second wind, aura of protection does more than indomitable. I know fighter subclasses tend to close that gap a little but it still is a gap. A lvl5-10 fighter after using 1 surge and 1 second wind doesn't have much else, usually relying on a powerful feat or two to replace powerful class abilities.

I want opinions on some potential buffs to the Fighter, or info from anyone who buffed the Fighter and how it worked out. 

I'm considering: 

• Adding a second action surge that only comes back on a long rest. (Level 7?)
• Giving 1 maneuver with 2 1d4 superiority die. (Battlemasters instead get 1 extra die of normal size at some point or something)
• Doing nothing, leave Fighters alone. Feats, subclasses and extra stat improvement/feat have it covered. 
• Fighters with a melee fighting style can sacrifice the normal benefit of that style to remove the disadvantage from being underwater. 
  for example: A fighter with great weapon fighting can use his maul underwater, but no rerolling 1s and 2s.  
  Normally wouldn't do much for a duelist, who can use shortswords and spears underwater, but if you had a magic battleaxe you wanted to use underwater this would help.