Fight Smarter

In 3.5, I think the duelist applied his intelligence modifier to his armor class

Depends on the GM and context. Could be considered meta gaming because your char may/may not have access to the information needed. Intelligence DNE knowledge.
If your char is an alchemist/equivalent? I don't see why not.

>CREEEED

Fuck, after all these years why haven't I made this an actual ingame ability. Maybe even multiple characters, depending on margin of successess or whatever mechanics the game in question uses to grade skill rolls.

I feel like something like the rangers favored enemy should be INT based. Knowledge about how to fight an enemy, where to properly hit, is something you can learn and something you can apply in battle. If we assume that INT also means wit, a sharp mind, initiative and reflexes could also be based on INT.

Play OSR games where fighters aren't kneecapped by 2+Int skill points/level

Definitely some kind of counter-attack ability.
>Cunning Counter: Once per round as a free action, whenever an enemy adjacent to you makes a melee attack, you can make a basic melee weapon attack against it first.
>You use your Intelligence instead of Strength for the attack roll, and no stat for the damage roll. If you hit, the enemy's interrupted attack has disadvantage.
>For each previous time you have used this ability in the same encounter, you take a cumulative -1 penalty to the attack roll.
I tried to be very conservative here because I have no idea what else you want your INT-based fighter class to do.

The intelligence not to get into a melee fight when they have a ranged weapon available.

Actually, the more intelligent fighting strategy is to keep your opponents at range unless it's necessary to engage in melee or you have a much greater advantage over them when engaging in melee.

And the greatest fighting strategy is to not fight at all if you can help it, instead goading your enemy into depleting their resources through feints and misdirection, attacking them through means they have little ability to fight back against, and engaging in battle only when necessary or overwhelming advantage is afforded you.

If such knowledge exists in the setting, I'd say you just need the requisite materials, construction equipment, and knowledge resources. So you'd need to acquire the right books on how to manufacture every component, mix and acquire every chemical, and all the right workspaces to build it in. You won't be able to build a bomb out of an old helmet with an unrefined sulfur rock, a bag of sugar, and a short length of rope in the middle of a wet forest, but you could with access to a basic alchemical library, a basic foundry, and a store of basic reagents, limited to the supply of iron/chemicals/requisite knowledge on the topic, of course.

>If we assume that INT also means wit, a sharp mind, initiative and reflexes could also be based on INT.

Well, the act of moving is DEX for sure, but the awareness and realization that you are in a situation where you have to move can be based on intelligence, in the sense of a sharp mind, the processing power of the brain, to analyze a situation and react accordingly. You know, how you can consider someone slow, not in the sense of moving slow physically, but moving slow in the mind. Though one could argue that this is also a WIS thing.

You realize I could think of a way to make INT reasonably apply to every interaction because it DOES affect every interaction someone does if they have a brain, right?