This has been a topic of discussion since long before I picked up my dice - casters have always been universally more powerful than martial characters in D&D. In prior editions, this was largely offset at early levels by their fragility - without a fighter or two to hide behind, no wizard ever made it to third level. But fifth edition, with it's "bounded accuracy" breaks that paradigm - wizards are somewhat more fragile than fighters, but with bladesingers, clerics in heavy armor, and valor bards, it's trivial to make a quite functional level one party entirely out of caster classes. And since martial characters contribute far less to every type of important encounter at level 11, that's exactly what a lot of groups do.
Now, many of you aren't really worried about class balance, and I understand that point of view. If you're building a character to roleplay, and combat is just part of the roleplaying experience, then it doesn't really matter if your fighter isn't as useful at higher levels - he's still the character you wanted to play, and you can milk his relative helplessness for all kinds of interesting character interactions. So if that's the way you play, you can probably skip this article - I (and much of my group) play D&D mostly as a tactical game with a story, and I'll be writing from that experience. Your experience of the game is totally valid and probably not affected by the 'issues' I'll be describing here.
For a while I labored under the idea that the problem was with martial characters - they weren't interesting enough, they weren't powerful enough! I tried to solve the problem by adding abilities, by forcing them to the forefront and elevating their effectiveness - this.. "worked," but produced a game very much like one that didn't contain fighters at all, but only different types of wizards - some of them wore plate armor and cast more melee spells, but they all play pretty much like a caster. In pathfinder, you can see some of the better attempts to solve this problem this way in the Path of War series by Dreamscarred Press - it added martial characters that were fun to play and interesting to build, but it dropped the fundamental separation between martials and casters just like my solutions (only it did a better job of it - if you play Pathfinder, I encourage you to check it out sometime).
But in fifth edition, there are numerous problems with trying to solve the problem that way:
So let's think about what martial characters are like in novels, and why people want to play them. I see several approaches to dealing with this problem in the genre:
Given that analysis, I'm going to try to expand the powers of Martials in a different way. I'm going to add martial-only feats and feat-chains (which are mutually exclusive with the Spellcasting feature) that are far more impactful on the flow of combat than their non-martial cousins. These will be on the same level as the battlemaster's built-in superiority dice feature (not the softened version that is already a feat), but will be roughly as varied as the existing feats are; intended to allow martial characters to specialize further for greater impact on specific types of combat and non-combat situations. They will also often have minimum levels, unlike most normal feats, since martial characters are already at a pretty good place at the lowest levels.
You can see my expanding list of homebrew Martial Feats here.