Considering how infrequently Mojang adds new metals to the game, (That is, never; until the Nether Update and Subsequent Cave update), it seems obvious that copper is going to be used for more than lightning rods, spyglasses, and aging blocks. There are countless different ways it could be applied- and one thought that occurred to me was a new means of defending your base from both monsters and other players.
Summary ~ An interesting new application for resources in the upcoming Cave Update might be the ability to make a sort of defense turret block themed after a shulker. This block could be armed with resources in order to shoot projectiles at nearby hostiles. These could be used to defend your base, and possibly villages. I think this goes off of the existing themes of copper being used for unusual devices, villagers being defended by mechanical warriors, and shulker shells being very, very useful.
To be clear, there are already plenty of ways to defend your base- but iron golems are somewhat too slow to be effective, and especially when playing with other players, it can be tricky to protect your base if you aren't online. Villagers are more or less completely defenseless in the event of a zombie raid or siege, even when the spawning mechanics for Iron Golems have (finally) been more or less fixed. I'm fairly certain this idea is far from balanced- but it is just that- an idea!
Function ~ The Shulker Turret, (Or perhaps simply 'turret'), is a redstone block that fires projectiles at enemies in a medium sized radius. It can be filled with arrows of any type, as well as fire charges, snowballs, eggs, and fireworks, (Though this would probably be more for effect than anything else.) When a hostile enters a certain radius, the turret opens up like a shulker box, and begins firing its contents at enemies. The Turret's cooldown is slightly less than a players, and it can fire projectiles fairly rapidly. It only fires when the target is within its line of sight, (unless equipped with fire charges), and fires on the nearest target if multiple are in range.
When it receives a redstone signal, the turret is 'locked,' and cannot fire. (In this manner, invaders could attempt to disable a Turret with a redstone torch.) More intelligent mobs such as skeletons will avoid turrets, and creepers will try and destroy them. Turrets can be refilled with projectiles via. Hoppers. When out of ammunition, the Turret also locks.
If broken without Silk touch, the Turret drops its ingredients and projectiles rather than its actual block for reasons that will become apparent later.
Crafting & Balancing ~ I'm not entirely sure what a recipe for such a turret would be- mostly because the recipe very much determines how difficult they are to obtain and how late-game such defenses should be. My first thought was that a turret should require two shulker shells and a dispenser in the middle column, (Similar to a Shulker box), surrounded by two copper blocks and two amethyst crystals. However, this would make them very difficult to obtain- not only would you have had to reach the End, but you'd need a reliable source of amethyst, which is apparently going to be fairly difficult to find. A more balanced recipe might simply require a dispenser, copper blocks, and redstone- though this contrarily seems like it would make them too easy to obtain. I'm fairly sure there's a balance somewhere, and I'd appreciate feedback in regards to this- (And the rest of the idea, of course.)
Turrets would need to be somewhat finicky to craft, if only because they essentially act as another endlessly vigilant player armed with a bow. The idea would be to encourage players to place them strategically around their base once they have access to the necessary ingredients, rather than simply surrounding their base with a ridiculous amount of turrets. (If this was really an issue, it could be made a feature that turrets remain locked if within a certain number of blocks of another turret.) While turrets also need to consume projectiles as a means of preventing them from becoming overpowered, it might be an interesting feature if they consumed less than players- however, ultimately, the turret shouldn't be more powerful than a skilled archer.
In addition, turrets would act as a moderately decent light source- which might also serve to alert players to their presence. While one hostile turret isn't much of a threat, several turrets firing at you at the same time quickly become pretty dangerous. It might be necessary to allow turrets to break blocks in order to prevent players from simply digging under them or erecting walls.
Other Related Ideas ~ There are several other aspects of turrets that I thought might be interesting, but perhaps too experimental, (As if the rest of this wasn't pretty experimental), to include in the main bulk of this thread. They are as follows:
Different Turret Configurations ~ Replacing the dispenser with a dropper in the recipe might make a bomber turret that can launch TNT, (within a smaller radius, so as not to render TNT cannons useless), Replacing it with an observer results in a Laser Turret that takes longer to fire, but remains locked on a single target, firing a guardian-style attack that deals a high amount of damage if the player cannot find cover in time- the laser turret might require an unusual source of fuel in exchange for being significantly more powerful- maybe it burns resources, like a furnace? Finally, using an ender chest would allow the turret to draw projectiles from your ender chest, at the expense of a slower reload rate.
Targeting Mechanics ~ Especially on multiplayer servers, it would be important for turrets to know what and/or whom they're supposed to be shooting at. A solution I thought of was to use an adjacent mineral block to tell the turret what to fire at. If the turret is adjacent to a copper block, it fires at hostile mobs. If adjacent to an iron block, it fires at any hostile mob or player other than the player who placed it. A gold block adjacent allows the turret to remember multiple players as friendly- interacting with a gold adjacent turret while it's locked causes it to remember you. Lapis would have the turret only fire at players, (Useful for minigames, possibly?) and a diamond block would cause all nearby friendly turrets to fire on whatever/whomever you last attacked or last attacked you. (An interesting side effect of this system is that pistons could be used to move mineral blocks adjacent to a turret, therefor changing what mode the turret is in.)
Turret Upgrades ~ This one is pretty simple- using different mineral blocks, (Iron, Gold, Netherite, Maybe Diamond) instead of copper to craft the turret results in turrets with a better firerate and more inventory. Of course, the whole idea of this was to provide a use for copper specifically- and if you combined upgrade able turrets with different turret configurations, all of a sudden you've got 12 different turret types that might look pretty similar to dyed shulker boxes, and it seems a little needlessly confusing.
Anyways, I'm (fairly) certain this is an original suggestion, and I think it would offer up some interesting new mechanics to the game- ideally without being overpowered or rendering existing traps and defenses obsolete. This could be applied in several different ways- from offering more protection to villages, (Unless you're mean and decide you'd rather steal all the arrows, I guess?)- to making new minigames and base defense options possible.
I like that idea, though I think it should fire projectiles the same way as dispenser does - and mind that arrows deal way less damage from dispensers than hand-held weapons - except being self targeting and repeating shots on its own every second.
I think redstone should activate rather than deactivate it as well.
I like the concept of targeting, though I would not include diamond block and replace it with something else, like emerald, amethyst or quartz block.
As for TNT launching - I think it's a bad idea, it would have too much destructive potential, especially self-destructive.
Ender chest loading - no, ender chests are personalized safe storage and could become too exploitable.
Laser variant seems quite powerful, maybe even unbalanced, though I do not oppose it directly so far, as it sounds interesting and balance-able
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Totalitarian architecture. Angular, defensible masonry over sleek, sloped carpentry.
Taking it slow. Provoking into attack typically does better rushing.
Underground engineering. Well-lit tunnels are much safer and often cheaper than any surface construct.
Farmability. One of primary concerns relating to any feature.
Practical mobs. No annoyances taking up space and processor power.
Generalism and simplicity. Overly complicated and specialized concepts hardly ever come into fruition.