Hi, 
First of all, sorry for any mistakes, I’m from Brazil and I’m not sure about the quality of my english xD.
A few suggestions about the item system:
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The monster within the 5 lvl gap up or above would have the full % for droping the item, if the gap is higher, the % needs to be lowered but only for equips, basic items that only drops from that monster needs to be full % all the time (when I mean full it’s like, if an item has 50% drop chance, if you are not on the 5 lvl gap, would lower to 40, 30, 20% ~).
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All items needs some variation, not only the gap between sets is huge but the variation is not good, example a Glove that gives Dex should also drop with another stat and a variation number like 9~11 Dex, or Int or Str.
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Now, another idea to fill the gap between sets is to create a second type of refinement system, for an example, you have the set for lvl 15, you would be allow to upgrade the same set reforging it for the lvl 30, the stats would go up. Each set would receive like a Rank II that would allow the user to upgrade one time in that level re-crafting it, just one time, just to fill the gap between levels.
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Yet another idea is to increase the possibility to drop the equip for the class the player is using, for example, one monster that can drop a staff and/or a bow, if you are playing with a class that uses bow for the main weapon you should have more % of chance to acquire that item.
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About the re-crafting the items another option could be roll the attribute, let’s say you acquired an awesome pair of gloves but it gives Strength and you’re a Mage, normally you would sell it but if you could re-craft the item only changing the attribute would be awesome and also would help to fill the gap between sets. In this system the item would only be possible to re-roll one time to another stat, and to roll to each stat would need a different item when re-crafting.
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The final suggestion is about the gems, you should be allowed to upgrade them, for example, you have 5 red gems of 1 star that gives some stat but you lose on another stat, by using 5 of that red gem you could create a Rank II 1 star Red Gem where the stat would go up but at the same time you wouldn’t lose nothing on another stat, each Rank II gem upgrade would need a number of the same gem and a catalyst, making more exciting to use the gems since the beginning of the adventure and of course creating more things to do in the early levels while helping to fill the gap between sets.
Well, that’s all, would appreciate another players giving the feedback about my feedback xD.