What about... energy?
Terraria is crazy mix of magic, technology, shrooms and tentacles, where everything is possible. While there are billions of ideas which are not (yet!) in the game, it's hard to implement something multifunctional that will be useful in every aspect of the game, except for general mechanics changes, which can be most difficult to implement, and propably that's reason for which there are not that much suggestions related to these.
However, when team is up for making such things, there is one mechanic that has not been implemented, which is energy.
It could be either of magical or technological nature - a some kind of mysterious power to run strange machines on it, or electric energy to create advanced mechanisms. The difference between them is only theme on which machines would be based.
So, one could say: how does it work then?
Just like in real world, players would need to gather energy in various ways and store it - not directly, though, but in special devices like power boxes or magic crystals. It could be moved around in special (or normal) wires, delivering it to machines which would use it to do various things - starting on amplifying effects of normal mechanisms, like reducing cooldon between shots in traps, increasing it's damage, and - in case of our beloved Flame Trap - range, and ending on being used as a fuel for special endgame machines, which would require it in order to work. Of course, using this energy to power various devices would consume it, so that players would have to create/find new, more efficient ways to produce and store it.
I personally think that making power plants and wiring them to various things to gain in strenght would be just fun.
A bunch of more precised ideas:
Both storage and gathering methods could be tiered - the farther player goes in the game, the better devices to gather and store energy he could make.
In my opinion, it would be best to make tiers out of various materials - lowest tier would be copper/tin storage/generators, while luminite - highest. It would also give more uses to this supreme endgame... metal? I guess it's metal, though I cannot be sure as for thing dropped by giant, flying lunar octopus wizard.
While conduits, which would transfer this energy would be placeable, storages could be both stationary (blocks) and mobile (items).
This energy could be generated in thousands of ways - from normal water, air and thermal/geothermal generators for technological, electric energy, to killing mobs or performing various magic rituals for arcane power.
Examples of uses, items and such:
- Endgame device using energy for terraforming (like excavating extreme amount of land, or filling caves)
- Ultimate biome control method, using energy to spread it or purify (inspired by purification machines shown in Terraria: Otherworld trailer. Thanks, Re-Logic!)
- Upgraded teleporters, which wouldn't need wiring to work, but instead would be linked together in some other way (frequency, code), and consume power to teleport player
- Amplifying existing machines, like reducing cooldown between statue spawns, or increasing traps firerate
- Building device which would copy actually constructed structured when energy and materials are provided (inspired by Supreme Commander building templates method)
- Custom engines/timers, with pulse frequency set by player with 0,01 sec. accuracy. Consuming energy per pulse would make them pretty balanced
- Superior light, consuming energy to light up enomrmous area
Now, if someone got to this point... I fully realise how difficult would it be to implement such mechanics. But when Re-Logic is taking challenge on their shoulders, such suggestion may not pass through unnoticed. Main reason for which I consider it to be such amazing thing is that it has a lot of uses, theoretically being usable with everything that exists in game world, even dirt blocks - for example, using such energy to frow grass on it... faster. Yeah, I know about Staff of Regrowth, it's just an example