Scaling/Leveling Items

Howdy!

Much like you have a love/hate relationship with WoW, the DM-hosts have a love/hate relationship with 4th Edition. 4E was the most WoW-like; it has its fans and detractors for this reason. I do not myself know much about it, but you may well consider that for your system.

I don't exactly follow your scaling logic. D&D, for all it's fun & games, is somewhat inflexible when it comes to scaling encounters to greater or less difficulty. To put it politely, the suggested encounter levels range from inaccurate to laughable. Back in the 3.5e days, this meant that from level 12+ the game became very different as every encounter became a contest of save-or-die attacks & massive damage. Giving your players over-powered weapons does little to help them survive a boosted encounter that will come down more to luck & initiative rolls than choices the characters have made.

Part of the Encounter Level estimates assume that the PCs have some amount of wealth, treasure & equipment. You could easily estimate the 'cost' of the weapons, in terms of bonuses (it's pretty easy to calculate) and deduct that from the average wealth per level of the players. They would need to make do with fewer non-weapon resources, but you wouldn't need to re-calculate anything.

Your approach seems to have a basis in WoW where magic users hold a weapon that applies bonuses to spells or direct damage or whatnot in a way comparable to melee weapons. That really doesn't have a good parallel in D&D-- there are very few weapons that have any effect on spellcasting capability. You will find plenty of people saying that non-spellcasting classes need a boost and your approach might be a solution.

As far as making the encounters more deadly or diverse, there are numerous ways to accomplish this. You don't even need to resort to making the encounter higher level. If all the challenge and capability is built into weapons, encounters become DPS calculations and contests of strength. A diversity of encounters should include radically different settings-- stealth encounters, trap encounters, diplomacy encounters, escapes, chases, navigation, treasure hunts and so forth. None of these really involve weapons, but the qualities associated with the class abilities and skills. The DMG should have advice as for how to make encounters memorable; you can take the same set of creatures and put them in different scenarios or settings. It's the difference between fighting 4 goblin in a flat dirt arena to fighting 4 goblin rangers in hidden tree positions with poisoned arrows.

This has become a fairly negative post, so let me offer some resources.
The 3.5 system had an expansion that included gems & sockets for weapons. A weapon had a 'slot' that could have the gem added with many and various effects like you have in mind. You needed a higher magical bonus (+1 - +5) to accommodate larger gems/larger bonus.
Also in 3.5 was the Kensai class. This was a prestige class built around the idea of a 'signature weapon'. The weapon grew in power and abilities with each level. There was a mechanic where the user could touch any masterwork quality weapon of the same type (sword, axe, bow etc) and it would transform to the signature weapon. This might be what you have in mind.
Monsters with class level. This is the easiest way to adjust challenge. Give every monster a class or two in fighter or rogue. I promise that is more challenging & easy to calculate.

I think based on the above, you can use your superweapons without re-balancing the game. Trust me on this the WoTC actually tried to do a good job balancing 5E to keep it fun at many levels with the suggested challenge level. What you'll need to do is add creativity & variety to the encounters via flavor and theme. Like I mentioned above, you have a lot of options for tweaking difficulty. Myself, I like making elaborate lairs and landscapes to steer the PCs towards or add time pressure to encounters. When in doubt, just throw a creature that can sunder & destroy the weapons or disarm your heroes so they don't have the weapons. That will be a real challenge.