Personally I like items that give characters unique abilities. In that way the always remain relevant.
To be clear, I don't think EVRY single magic item you find should always "remain relevant" across the entire game. There should obviously be some degree of variation in quality.
But I'm a strong advocate for:
- making the gap between tiers relatively gradual and marginal. The difference between +1, +2 and +3 weapons in D&D is a perfect example of the concept, but not necessarily the only way to go.
- having a limited number of tiers. There should be early game/low value items, good ones, excellent ones... Maybe legendary ones too. But that's it. If you find something that "seems very good" it should stay relevant for hours and maybe even become part of a solid build, not be disposable after every level up or so.
- making the unique "secondary effects" of your items more relevant that the numerical ramp up in power. As from a previous example: moving from "+1 damage" to "+2 damage and does double damage to undead" is a lot more compelling than "+2, +3+, +4, +5, +6" and so on.
As for passive bonuses, I'd love the ability to upgrade items to keep them relevant to my level.
Might & Magic X Legacy had something like this and it worked fairly well.
The game had a baseline of randomize loot ranked in more or less 5-6 tiers of quality (can't remember exactly) and then every now and then you could loot golden pieces of equipment called "Relics" which had the interesting characteristic of starting as basic items in power (they were at the level of a good early game magic item and nothing more) and then rank up in level for three more times. At level 2 they were decidedly better, at level 3 very strong and at level 4 they became basically "best in class" in their respective category of weapon/armor.
Of course as a result by the end game they were basically the only items that really mattered for your party and every non-relic item was just a placeholder/vendor trash waiting to be replaced.
