There are a lot of good, extensive guides on the wiki that show really cool loot pools, but majority of these items turn out to be of zero impact as there are certain item sets to go for that are simply 'better'. With the more modern systems of Grading, Random Options and more impact from Refining on special gears, it feels like there's a lot of untouched gold so to speak.

That said, I'd like to suggest the following:

Enchanting Stones:

Enchanting stones(usable once, destroyed on use), that can be universal, just for weapons, or whichever. That add certain extra attributes(Random options), based on the quality of the gem. For example:
A quality 1 enchanting stone can potentially add 1-5 to a stat and has a maximum amount of options limited to 2, a quality 10 enchanting stone can add 35-50 to a stat and a maximum amount of options limited to 4, but always more than 1. Using these on already enchanted gear overwrites the enchantments.

Grading Stones/Set Stones:

Grading stones(usable once, destroyed on use), to add a grading dependant set of bonuses to an un-graded item.
Let's take the ol' pre-renewal favourite 2h sword, Muramasa for example. This weapon doesn't stand up to the newer weaponry at all, but it has some interesting inherent bonuses which builds were made around back then. With a grading stone you found in a high end dungeon, you can add a grading path to it, and suddenly it's interesting to grade a found muramasa, because the bonus, in combination with the grading, makes it a viable weapon.

How will these things impact gameplay?

It has the potential to make illusion gear, which is often skipped over now en masse, interesting since it adds elements of flexibility with enchanting stones. Grading stones in turn will make most now obsolete gear, interesting as well. Combining these opens the gates for a lot of developer freedom to make even more challenging content, custom weaponry, armors and whatnot, and give the players a new level of freedom to brainstorm new builds, new ways to play, and it exponentially increases the interest of the vast world of RO.