* Keep the same D&D magic level (magic items, spells) for players and monsters
* do not alter the class progression or rules
* do not make my content weaker or incompatible with official D&D content of the same level
* make magic items more special and worth hanging onto
* make magic items and spells special and mysterious again.
* do not have magic items or spells something that players can walk into a town and buy.
* do not have commoners with magic items or spells being used as technology or for comfort
1) Enforce a "spell rarity" concept. Players who want to learn a particular spell have to jump through hoops- they have to find a teacher, tome or scroll to learn from. You see this in older editions of the game, and in D20 offshoots like Arcana Unearthed/Evolved. You can't just learn any spell in the game...
2) Make spellcasters rare but powerful, perhaps by making the ability to channel arcane magic a question of genetics (Wizards are born, not made)- so there is a gateway Feat anyone to play an arcane spellcaster must take at 1st level. Like Magical Aptitude.
Divine Casters would be forced to take the (houserule) Feat Touched by the Divine to cast spells, but they could take it at any time, since they are being given a gift from beyond.
If you think a feat is too high a cost, make it a bonus feat for any spellcasting class- but make sure it remains a 1st level requirement!
3) Make Item Creation more difficult, either by upping costs, changing component rarity, or limiting the IC Feats. That way, you have ultimate control over whether the party has a particular magic item or not- they can't merely have their party spellcaster be the party's arcane armorer.
4) The "weapon of legacy" idea is sound. Even if you don't actually use those rules as is, make the items either "improvable" or "unlockable." That way, the PCs will have an incentive to keep items rather than trade or sell them.
Furthermore, the WoL system requires PCs (of any class) learn feats to get full use out of the items. There will be less overall power per PC because of it.
5) Zones of high, low, wild or no magic. When magic becomes intrinsically less reliable, the PCs will be forced to rely on other aspects of the world for victory. Their own blood, sweat & tears. Tactics. Bait.
There are lots of fantasy novels set in worlds like this- the first that springs to mind is Larry Niven's Magic Goes Away series.
6) Change spell casting and duration times. If spells take longer to cast but also last longer, things get more tactical. Harry Turtledove's Darkness books have very powerful mages indeed- remeniscent of Gandalf, Merlin, Elminster and other iconic mages of fiction and RPG fame.
But magic takes time- there are few if any true combat spells, nor are many of the spells they sling the kind of thing they can unleash in mere seconds. Yet at one point, one group creates the equivalent of a Necromantic Atomic Bomb.
If casting a Fireball takes 3 rounds, for instance, the rest of the party is going to have to work hard to keep the mage alive and unscratched (pesky Concentration rolls!) for one to go off.