The years I spent searching for the perfect system that doesn't exist. And by system, I generally mean, combat system.
Only to discover the OSR and the notion that the older systems are all you really need, i.e, tweaked of course, but a solid foundation. There's lots of cool little ideas and dice tricks in the OSR universe to spice up your games. If you care about mechanics. And I do.
D&D combat is illogical. That's not exactly news. It's always been an abstraction, one originally designed to simulate armies against armies, not necessarily man against man.
Two 1st level fighters should fight to a stand still. Yes, maybe one's a little stronger or one's a little quicker, but they're basically evenly matched. Their level is their fighting skill. Two 5th level fighters should also fight to a stand still as should two 10th level fighters, and so on. A 5th level fighter should whoop a 1st level fighter -- here the rules accurately account, if only because of hit points.
The main difference in these duels is that the 1st level fighters, though evenly matched, will see one of the fighters fall quickly do to a lack of hit points, were as the 5th & 10th level fighters toil on and on. Two evenly matched fighters of any level should toil on and on. So logically, should hit points change depending on your opponent? That's not gonna happen. If anything damage should change, which does happen somewhat.
My chances of defeating my opponent rest mainly on how good my fighting skills are compared to his. Yes armor plays a roll, but it only delays my pummeling of a lesser opponent. This dovetails directly into the notion of armor as damage reduction.
Armor Class. It makes perfect sense for ranged combat. Most people can't dodge arrows even if they know they're coming, so basically, distance and what armor your target wears are your primary obstacles to a successful hit. Shields should factor more. In fact, shields in general are way undervalued. If entering combat and I had to choose sword or shield, I would strongly consider choosing shield.
One of my favorite representation of man to man combat was in the DC Heroes RPG by Mayfair Games. A system referred to by some as, MEGS. The system is 2d10. On the Action table, cross reference your score vs. your opponent's score (usually Dex vs. Dex) to find the number you need to meet or beat. If you roll doubles you get to roll another 2d10 (I would consider changing this to simply rolling another 1d10 to avoid ridiculous, if not rare, outcomes.) Notice on the Action table that evenly matched foes of any power level have to roll an 11 to hit.
Then you cross reference your effect value (usually Strength) against your opponent's Body score (modified by armor) on the Result table, including any column shifts from your success to see the damage inflicted. But once again, a fight between evenly matched "regular folk" won't last long because of low health values. Still, it's elegant. Buuutttt...CHARTS. They slow the game down, or do they really? We never had problems with charts when we used them.
Charts were a thing in the 80's. By the 90's they were pretty much obsolete. The thing with charts though, is that they can provide fairly logical results for a system. Marvel's FASERIP system used charts well. The huge flaw in FASERIP though, is that your foe's fighting skill had no bearing on whether or not you could hit them. The chart simply existed to determine how well you hit them. Your average person, with Typical rank Fighting, has a 50% chance of hitting anyone.
Then there's Palladium. Strike, Parry, Dodge, Roll with Punch....! A very granular, opposed roll, chart-less system, love it or hate it. I love it. Ideal for one man vs. another. Five on five?... good luck with that. Palladium Fantasy uses the same system, but this type of combat takes far to long for a dungeon crawl (and ultimately, it's all about accommodating a dungeon crawl!) This type of system almost requires a comic book (cinematic) style of action narration.
Notice how in comics and movies, when groups fight each other, they focus on one or two characters at a time. You'll see a series of actions, strikes, and parries before switching to another character. Often, the results of the first little scene will lead directly to the next, for example, the next two combatants will move into the background of someone else's scene before becoming the focus themselves. This brings up a whole 'nother aspect -- initiative and turn order.
One of the reasons that D&D combat can be tedious and not dynamic, is that it's essentially a frame by frame narration, going from fastest to slowest. You go first, swing and miss. The scene immediately switches to the other side of the room where someone else acts. Then the focus switches again. You rarely get to see an immediate rebuttal from your foe. Not very exciting.
What if, you focused on whoever acted first for a couple of rounds of give and take, and then switched to the next person for a couple of rounds. Does everyone declare their intentions first and have to stick with them? Or do you keep it fluid and let people choose their actions depending on the events of those that went before? It gives everyone a bit of a spotlight for a few moments instead of the regular slow-motion chess game. Of course it causes problems for spell durations and stun durations and rules-lawyers would absolutely lose their minds! It would take a strong DM.
Another game system that had wild potential in my book is Iron Kingdoms. Love its use of derived stats and the 2d6/3d6 resolution mechanic has all kinds of potential for cool little dice tricks. But it's basically a glorified miniatures game, practically requires them. I would do away with the FEAT point system entirely. And magic would need modification as every spell is simply a different version of magic missile. Man, this game could have been it.....
Where have I gone with this ramble???
Back to mano-a-mano and D&D. Without rewriting the rules all together, the simplest solution for me has been to add a parry option. An active parry option, not a +2 or +4 bonus to AC for fighting defensively -- far too passive for my taste. You don't want to bog the game down with parries, so it's just an option. If you haven't already acted, you can try to parry an incoming attack. Meet or beat the attack roll with one of your own. Perhaps a bonus if using a shield (or advantage.) Then you can't attack that round. If you're playing your character realistically, they would always choose to parry if they could (unless you really embrace hit points as endurance and fate, which they kind of are.) Perhaps Barbarians (berserkers) don't ever get the option. It slows combat down a bit because there will be successful parries, but that combat is more exciting, a touch more real.
Just some stuff I always think about.
And then there's the quest for the perfect, non-Vancian magic system...