As I’m playing this game, the more and more ideas I have. Here are some of them:
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A set of harvesting tools that can be obtained when you can obtain the star metal tools (level 55). They have the durability and harvesting power of steel tools, but you can gather a lot more rare resources such as goldstone from obsidian, resin from northern trees, crystal from stones etc.
Or maybe add this ability to the acheronian tools because they are underperforming. You can learn them only at level 60 and have the same gathering power than star metal at level 55 and less than the obsidian kit at level 60. And they have low durability. Yes, it only takes hardened steel to repair, but at that level you should have a lot of legendary repair kits or a blacksmith who can craft them. -
A cripple effect when you take damage from falling that height. AFTER the falling bug is patched of course.
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A couple of other ideas I mentioned here: Bump to move your follower and here: Customizing characters +++ and maybe here: Making Corruption More Challenging
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Craftable compass and minimap, both can be learned from the archivist where you learn the cartographer. Both could be crafted at the artisan table. Compass would cost some glass and 1 iron bar, and the map should cost leather and inking brush and coal + water-filled glass flask (to paint the stuff onto the leather).
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A pair of… swimming boots? (sorry I don’t know what they are called) for faster swimming just like the climbing gear for climbing.
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Add shields to more enemies! Only a few of them have, and not even everybody’s actually using them. And some kind of dodge mechanic would be great for their health. Maybe a step-back or a sidestep similar to the previous player dodge mechanic.
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Make the nights scarier. At night more elite animals should spawn. And the temperature could change too: in the colder areas the daylight temperature should stay as it is now, but at night it should go lower.
In hot areas the opposite: the night stays the same but daytime would be even hotter. Not everywhere only in the snowy/mountain and savannah/unnamed city areas so fresh starters would not die instantly in the desert or at the river. -
A REAL marketplace at Sepermeru. You could change low-to-mid tier materials based on their role. Like building/smithing: X brick for Y steel bar, or clothing/decorating: X silk to Y leather, cooking/chemistry: X aloe leaves for Y puffball mushrooms, etc.
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(maybe too OP I don’t know I do not play PvP, but): Invisibility potion. Should require a fragment of power, dragonpowder and water-filled glass flask, alchemical base and it makes you invisible for 3 seconds when you consume it. Could be learned at the archivist after you unlocked the library of esoteric artifacts feat.
I definately have more, but these are the ones that came to my mind right now. 
