Mimics. Anyone with half a brain and any experience with Mimics in the past will be able to spot them a mile away. After the first encounter or two (or after reading the wiki page) they become less exciting and interesting. That's why I think that Mimics need a better system of mimicry. The old method isn't really cutting it.
Mimics should be placed tiles in the first stage, so that they're centered and ordinary looking like real Chests. Half the time they spawn, they are hanging of a cliff, or embedded in a wall, or otherwise out of place so that it's obvious that they aren't real. Instead of spawning like normal enemies, they would be generated globally in the same way that Plantera's Bulbs are generated in the Jungle instead of just popping up when a player is nearby.
For even further Mimic undetectability, there will be two new varieties. Any Mimic tile that spawns underwater will become a Water Chest Mimic, and any Chest that spawns in the Lihzahrd Temple would become a Lihzahrd Mimic. Lihzahrd Mimics would have a special loot table and unique drops.
Check out this link for further details.
Now, to make them more of a threat, this fake Chest tile would be labeled exactly how it normally would be on a map, so you can't use the minimap to find them. In addition, they would glow yellow to players using the Spelunker Potion to make them more Chest-like as well, and they're still a chest filled with money and treasure, after all.
Now for my favorite part of the idea. Instead of being revealed by Hunter Potion, it would remain undetected by its effects until they actually start to attack. Instead, the only way you would be able to find a Mimic (other than getting too close) would be to use Dangersense Potion. While Dangersense is active, false Chests will sparkle in the same way the Cobwebs do, which is a lot less noticeable as an effect, but still can be a dead giveaway to people who know what they're doing. Not only is it a cool screen effect for making Mimic hunting more interesting, but it would also have no clash with Spelunker Potion's screen effect.
That's it. If you agree with these changes, let me know! Feel free to leave comments. Feedback is appreciated.