I actively dislike +X magic items. The result is you get monsters with +X to saves and +X to AC and +X to ATK, and PCs who don't do the +X item treadmill get left behind.
The disadvantage of not maxing your ATK stat also compounds, as being -1 behind is less than half as bad as being -2 behind (etc).
So +X item treadmill just encourages rush-to-20 stat and inter-party power disparity and ... not much else. They aren't even that exciting, as they fade away into the numbers on your character's sheet.
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As an example:
A shield that can cast shield 1/rest is a bit better than a +1 shield. Each attack on you has a 5% chance of missing due to the +1 shield. Each attack on you has a 25% chance of being a chance to be missed because the shield can casts shield, assuming you still have a use of it and a reaction.
If you are attacked 40 times between rests, that +1 shield will on average turn 2 attacks from a hit into a miss.
If you are attacked 20 times between rests, that +1 shield will on average turn 1 attack from a hit into a miss.
If you are attacked 10 times between rests, that +1 shield will on average turn 0.5 attacks from a hit into a miss.
If you are attacked 5 times between rests, that +1 shield will on average turn 0.25 attacks from a hit into a miss.
If you are attacked 40 times between rests, that +1 shield will on average turn 0.99999 attacks from a hit into a miss.
If you are attacked 20 times between rests, that +1 shield will on average turn 0.997 attacks from a hit into a miss.
If you are attacked 10 times between rests, that +1 shield will on average turn 0.94 attacks from a hit into a miss.
If you are attacked 5 times between rests, that +1 shield will on average turn 0.76 attacks from a hit into a miss.
(this assumes follow-up attacks don't occur. If they do, the shield of shield gets better by 0.25 per follow-up attack).
Even a weaker boost, like +2, is pretty close:
If you are attacked 40 times between rests, that +1 shield will on average turn 0.99 attacks from a hit into a miss.
If you are attacked 20 times between rests, that +1 shield will on average turn 0.88 attacks from a hit into a miss.
If you are attacked 10 times between rests, that +1 shield will on average turn 0.65 attacks from a hit into a miss.
If you are attacked 5 times between rests, that +1 shield will on average turn 0.41 attacks from a hit into a miss.
This is already close to on-par with the +1 shield, despite the bonus being a mere +2, on a non-crazy number of attacks between rests.
The big difference is with the shield of shield, the player is doing something to make an attack miss, instead of not even noticing the effect. Higher bonuses just means you can more reliably save it for an important attack.
The player with a +1 shield probably won't ever really identify with the shield. The player with the shield of shield may be like "boo-ya, suck it DM" whenever they use it.
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The second bit of itemization is to avoid giving too much +damage per hit. Fighters go from being good damage dealers to insane damage dealers if you scale up the +damage per hit.
Give some of that. But don't make it insane. A 1d4/1d6/1d8/1d10 at T1/T2/T3/T4 is more than enough to make fighters drool (up the die size by 1 for 2 handed weapons to d6/d8/d10/d12).
Having +damage per hit once per turn benefits fighters more than other characters, but not to nearly the same degree.
With a 65% hit chance, a 2 attack PC gets 88% yield from a 1/turn tap. A 4 attack fighter gets 98%, 1.12x as much instead of 2x as much. Throw in crits and it is even closer (if the PC can pick which attack to use it on after knowing it crits it is slightly better).
What each melee class wants differs.
In T1 and T2, the classes are pretty similar, except the Rogue. In T3 and above:
Rogues want off-turn attacks. Landing a 2nd sneak attack is really nice. Weapons that give them reaction attacks are good.
Rangers ... well, they need ranger-specific boosts, because the base class is still lacking. Not much to hook onto.
Paladins can be given a sword that does nothing but make their targets 1 more step vulnerable to radiant damage and they'll be happy. (no plus, no extra damage, just vulnerable radiant).
Barbarians are tricky. A 2 handed weapon that does bonus damage once per round is, however, pretty solid.
A Monk would probably be very happy with a weapon that gives then a 2nd bonus action, but they even like per-tap better.
Fighters love per-tap damage.
A +X per tap item (doubled on crits) is worth
monk: 3.5
fighter: 2.8
barbarian: 1.95
paladin: 1.4
ranger: 1.4
rogue: 0.7
A +X per turn item (doubled on crits) is worth (assuming you use it on the first attack that hits)
barbarian: 1.08
monk: 1.04
fighter: 1.03
paladin: 0.93
ranger: 0.93
rogue: 0.7
You'll see how this is much, much flatter. The barbarian gets 1.5x what the rogue does, while the monk got 5x (fighter 4x, barb 2.8x, paladin 2x, ranger 2x) what the rogue does with the per-tap gear.
Another benefit of the per-turn items is that OAs start being painful again.
...
Anyhow, this is what I think about when doing melee character itemization.
The next thing is to lean into utility. Having items that do something instead of just boost combat ability.
The best status condition is dead. But boosting combat ability is, to a great extent, a treadmill; monsters scale to PC capabilities. It does make "old foes" fun to revisit, and players like optimizing.
But if you give items that grant interesting effects things can be more fun. For example, Nature's Mantle; it is a druid and ranger item that grants the ability to hide when merely obscured.
Do you know what obscures? Dim light, or darkness with blindsight.
That item is rather fun to give to a rogue|ranger; they get pseudo-invisibility as a bonus action.
Other items that are more than just combat number boosts are also good.
A belt of 23 strength is mechanically powerful; a belt that grants +1d6 damage on strength-based attacks 1/turn, +1d6 on strength saves and checks, grants the siege monster feature, and increases carrying capacity 10x... that in-story is the same item, but instead of being mainly a damage boosting item it now gives the player serious options, and more if the player is creative. What more, it becomes an item that is better on a strong PC instead of the other way around.
Items that give limited flight are also great in the hands of melee characters. "Windwalker Boots: You can choose to fly with a 60' speed on your turn. At the end of your turn you fall to the ground if you are still in the air. Once you have used this, you have to spend an action recharging it before you can use it again."
This can be tweaked up and down; it can take an action to fly, a bonus action to fly, no action to fly. The recharge cost can be an action, not moving for a turn, or a short or long rest. Each of these slight changes makes the boots better or worse.
Throw in a reaction based dodge for some dynamism "If you are hit by a melee attack or fail a saving throw, you can expend a reaction and fly 30'. This movement does not provoke opportunity attacks. If this takes you out of range of the triggering effect, you are no longer subject to it. Once you use this ability you cannot use it again until midnight passes."
Such 1/day or 1/rest items aren't going to dominate combat, but they will make for neat moments. Especially because they aren't just damage boosts, but instead safe your arse options.
And the ability to fly, even for a short distance, really makes a melee PC feel less sidelined in my experience.
Simpler stuff can also be a hit. The studded leather that lets you change what it looks like is amazing in the hands of a creative player.
Boosting "lower tier" armor is also fun. A +1 on a scaled armor is just fancy half-plate; so you can toss something else on it.
The "free" AC bonus you can give on armor, then throw in an extra property that is not just "bigger numbers", is:
Plain Leather: +1
Hide: +2
Chain Shirt: +1
Scaled: +1
Ring Mail: +4
Chain Mail: +2
Splint Mail: +1
For heavy armor, those are all weaker than mithral plate armor (which also removes str requirement and disadvantage on stealth).
Giving up AC is painful; so unless the item is crazy good, players probably won't do it. So use the above to give +X vanilla items that don't cause AC-inflation while also granting some other magic property that makes them better to wear than the mundane higher tier armor.
