It’s too bad that TSR got hung up on inventing more and more wizard spells. AD&D added three more levels of spells compared to Expert. All the supplements, as well as adventure modules and Dragon Magazine, were full of them. Meanwhile, fighters and others never got any cool new stuff. This created a huge gulf between casters and martials that remains to this day. But imagine if we got bold and flattened wizards and other casters out and gave martials some cool stuff….

Spellcasters

AD&D spells up to 7th level would be spread out over 15 levels, & that’s the maximum class level. Now, when you reach 8th level, you have access to “8th-level” spells–which are actually the less powerful entries in the old 4th-level spell list. This means that instead of getting fireball at 5th level, it would now (being among the more powerful 3rd-level spells) be a 7th-level spell, which you couldn’t get until you reached 7th level. But lightning bolt would be a 6th-level spell.

The way to do this is to split 1st-level spells into 1st-, 2nd-, and 3rd-level spells, requiring some additional weak spells (there are plenty to choose from, even just in the 2nd edition PHB). Then 2nd-level spells become 4th- and 5th-level, etc.

We can keep most of the old 8th- and 9th-level spells as “legendary magic”. These would be reserved for powerful monsters and NPCs, as well as powerful magic items.

Non-spellcasters

Then let’s say fighters and thieves got some feats as well; keep it simple.

All warriors features

• Steely Nerve: At 5th level, you are immune to non-magical fear, phobia, and morale problems. You get advantage (per 5e) on saves against magical fear and similar effects. This also gives you an air of authority; check CHA to get friendly or neutral NPCs up to 4th level to do what you (reasonably) want.
• Multiple Attack: At 7th level, you get an extra attack at disadvantage (per 5e), which becomes normal at 13th. Specialized fighters get this at 1st and 7th and a third attack at disadvantage at 13th.
• Grand Charger: At 14th level for a fighter or paladin, and 11th level for a ranger, you can go on a quest to find a semi-intelligent fantasy mount, such as a griffon or giant panther.

Fighter-only features

• Warrior’s Instinct: At 3rd level, with a shield, you can sacrifice your shield to block an attack on yourself or anyone within 5 feet after damage is rolled. The shield must save vs 7 or be unusable until it can be repaired by an armorer. Without a shield, you can spend 2 hit points to act as if you had an action readied for the round; this is not an extra attack.
• Battle Cry: At 8th level, you develop a battle cry that, when shouted as you close for melee, gives advantage on the next attack to all allies within 30 feet, including you (once per combat).
• Horde Bane: At 11th level, when facing at least three of the same type in melee, you treat creatures of 2 hd or less as if they had just 1 hp. At 16th level, 4 hd or less.
• Defiant Spirit: At 14th level, you can draw on your heroic spirit to cut the duration of magical effects on you in half and make saves against magic with advantage.

Thief features

You gain a point of luck every time you roll a natural 1 or a natural 20. You can spend luck on other features (below) or use it to reroll a die roll. (You can’t reroll more than once or reroll a reroll.) At the end of the adventuring day, you lose all unspent luck.

At 3rd-level, you start every day with 1 point of luck.

You pick up skills haphazardly: at 5th, 8th, 11th, and 14th levels, the DM should offer two random choices you don’t already have. Once when offered a thief feature, you can instead choose a fighter or warrior feature of equal or lower level.

• Evasion: When you are hit in melee but before you know what damage it would do, you can spend a point of luck to evade the blow, but you end up several feet away… (1d12: 1=DM’s choice; 2-10=clock positions around opponent, 11-12=your choice).
• Silver Tongue: Given the opportunity to spend adequate time with a person you can communicate with, you’re so persuasive that you effectively have the ability of Charm Person by spending a point of luck. If successful, it lasts 1 day.
• Coup de grace: If maximum damage from your backstab could kill a bloodied opponent (hp at 50% or less), you can spend a point of luck, and, if you hit with a normal attack, kill the opponent.
• Swashbuckler: For 1 point of luck, you can tuck and roll after a fall (up to your DEX score in feet) to avoid falling damage.
• Viper Strike: You can perform a backstab-style attack face-to-face, as long as it is the very first attack of a combat encounter. You create your own surprise.

Combat

Combat should be altered similarly to the way I fixed 2e.

The Result

This would radically realign the classes to make them more equal and, I feel, make high-level play more viable. As a trade-off, I would say wizards could have d6 hit dice and slightly more spell slots, so they’re more viable at low levels.