Seems this is my first post since the Official forums were created. I've mentioned a few of these on the Terraria online forums so a few of these may sound familiar.
Statue Expansion:
New Mob Spawning Statues for a few enemies.
Penguin Statue - Just like the bunny and goldfish statues the penguin statue would spawn penguins or under the blood moon the corrupt/vicious penguin variants. Like the bird, bunny and goldfish statues the main if not only function is for banners. (Or target practice for some people.)
Antlion Statue - Following the 1.3 update and the inclusion of the new Antlion enemies this is less relevant than when I first suggested it. However, Antlion mandibles and the associated banner are still sought items. The major drawback to this is the ability to farm sand from the summoned Antlions. Though, given the statue spawn cap and the slow speed of sand creation I feel this is a smaller concern. (Besides, is lots of sand really game breaking?) Again, post 1.3 this statue is more of a novelty than a necessity.
Hornet Statue - The primary purpose for this statue is stingers. As most players would agree these are very common in normal mode as a result, this is mainly for hardmode. Specifically because, for whatever reason, moss hornets do not drop stingers and they greatly outnumber hornets once hardmode starts. The value of stingers being, use in the Abeemonation, Thorns potion and Flask of Poison. The major complaint this suggestion got when I first suggested it was ease in obtaining jungle gear for new players. So I will make my case for why it isn't an issue. First, to even have a chance to farm hornets a player must locate a hornet statue which I would wager takes longer than simply killing the plentiful normal mode bees unless the player is very lucky. With the luck needed to find just one, a farm of multiple is highly unlikely. Second, to build a farm a player needs wire, and a wrench both of which require a mechanic, which spawns after entering the dungeon. At this point the player likely has jungle or superior gear anyway, and definitely has access to superior gear. Finally, the jungle gear requires either vine or jungle spores or both, items which would force the player to enter the jungle at which point they could farm stingers anyway. In short, I truly believe the best use of these statues is for getting stingers after activating hardmode. The best part of this statue is it already exists.
Demon Eye Statue - Like with the Hornet this statue is mostly for hardmode. At least that was the original intent. However, with the 1.3 update wandering eyes now drop both lens and the Demon Eye banner. Making this statue less needed and more of a novelty. (Eh, it was in my original post so I thought I'd at least mention it.)
Spider Statue - Now I want to make it clear that this statue is for the wall creeper and not the black recluse. The function of this statue is access to the spider banner after reaching hardmode. This is due to the wall creeper being completely replaced by black recluse once hardmode is reached. But, since the last time I suggested it I came up with another use. Spiders outside the spider walls, specifically for player made dungeon structures. Think of how fun it would be to build a trapped room that drops spiders on people. I strongly suggest the statue remains wall creepers not only for the banner in hardmode, but also so that new players don't encounter super strong enemies in normal mode.
To sum up, the statues suggested serve the primary purpose of banner collection and the ability to get materials that become scarce (or vanish) in Hardmode.
Mechanical Triggers:
New ways to start devices.
Solar Switches - Quite simple On in the day, Off at night. There are two ways to do this. First, require direct light from the sky. Meaning they would need to be on the surface. Second, just have the switch occur when the game changes to night or day.
Pressure Switches - Another simple concept, (though I don't know how simple to add) On when stepped on, Off if nothing is on it. This is different from the Step once for On, twice for Off that is already in the game. Makes it possible to create actuated traps without the need to reset after use, or complicated wiring. (Certainly, if there is an easy way to do this already I am curious to hear how.) It could come in player, and enemy variants but, it might be cool to have any weight do it, such as blocks (or boulders) for breakfast machine type devices. The one question this raises is how fast the pulse is sent (IE 1-sec Timer) while continuously pressed. To deal with that I figure variants could exist (1 sec, 3 sec, 5 sec, other) with increasing requirements to make/buy.
Proximity Switches - Much like the Dryad's shield, when a player or enemy enters the switch radius the device will activate. The purpose of this is the ability to hide switches more effectively than with pressure plates. (Which fool enemies and sometimes work in the right places but otherwise are very easy to see.) It would sense players even behind walls, making player traps very hard to detect. (For the record, I am opposed to including these switches in world generation near explosives...)
Traps:
To conclude this post some ideas for traps in general.
Building Traps - Like it sounds, crafting of current traps. For example poison darts + stone for Dart Traps. For the temple traps it would require Lihzahrd block and could use the Lihzahrd Furnace. The ingredients and quantity are up for debate but personally I would gladly spend a full flamethrower to get Flame Traps rather than go from world to world cleaning out temples and getting 1-2 at a time. Spike Ball traps I figure would use the spike balls from the goblin invasion. Spear traps would probably just use iron/lead since there are few economically craftable spears.
Dungeon Spike Ball Traps - Roughly at the same level as a standard dart trap, these would damage like goblin spike balls and function as an early game equivalent for the temple Spiky Ball Traps. Unlike spears and flamethrowers, spike balls can justify an early game version due to the existence of the goblin spike balls. These would exist naturally in the dungeon but, could be crafted with stone and spike balls.
Anvil Traps - Who wouldn't want to drop an anvil Looney Tunes style on another player. The trap version of the anvil would require more iron/lead than the standard one. In exchange it can be placed, or can be dropped mid-air. The anvil then falls like sand until it hits a solid block. To make this work effectively it would need to be 1x1 so that it can always land. (Unlike tombstones that require 2x2 or they become unobtainable.) Unlike sand and friends, the anvil does not cause suffocation but instead causes heavy damage. (More than a boulder, but in exchange it can't roll.) Of course, these can also be set in places like boulders are for pressure traps. (Or even a breakfast machine...)
Toxic Trap - In short, a trap that causes large weak explosions. (Non-damaging to blocks) Similar to the toxic flask (and likely crafted from it) the Toxic Trap simply fires a toxic cloud to the left or right of where it is placed that explodes on impact. (Block, player or enemy) the explosion causes weak damage and has a chance to poison. Do note, that when I say weak I mean compared to explosives. It would still be stronger than all but temple traps. May be fun to have both a dropping and firing variant to allow attacks from the side and from above. This traps would be crafted only, never spawning in the world.
Hopefully, these are worthy suggestions. Feedback both positive and negative always welcome. (As long as it is polite) I do apologize for the massive wall of text, I figured it was better than several posts back to back. (Took ~2 hours to write) Enjoy.