How about a new fluid, since we missed out on getting a fourth one in 1.3? Pink-colored "Capacitance Gel" (made partly from Pink Gel, with other more late-game ingredients, Conductive Alien Goo from Martians perhaps) which can act as a conductor if wired. For instance, a switch with a wire extending into a pool of it, and on the other side of the pool, a wire extending out. Flipping the switch would allow the signal to travel through the gel to any other wires overlapping connected fluid. A good signal multiplier without loads of complicated wire networks.
Also emits light when a current passes through it, which lingers but slowly fades. (1-second timer should be able to keep it lit up permanently, 5-second would fade completely before cycling.) Players and mobs inside the fluid when a current passes through would be damaged. This would allow for safer and more controlled "lava moat" type constructions.
Not as viscous as honey, perhaps the same as lava. Wouldn't be pumpable I guess, since the conductive mechanics of the fluid would probably mess with the pumps, but being craftable to start with means you can always get more if you want it. (Recipe should be fairly generous in output.) Also, the first craftable fluid! Able to be picked back up via buckets, and for easy clean-up, should be able to be removed by the Super Absorbent Sponge.
No idea how it would interact with the other fluids. I'm assuming that the game isn't designed to allow fluids to actually touch long-term, which is why all fluid interactions form solid blocks? Maybe some aesthetically interesting or useful blocks could be formed from the interaction of Capacitance Gel with the other three.
Also of course, you should be able to fish in the gel. Maybe some crazy fish you can catch for crafting an Engineer Potion for faster/longer reach while laying wire or activating inputs, and maybe an Electric Eel which could be a new weapon as well as a tool that could signal wire directly, without a placed input like a switch or lever.
(Demolition Man reference too, and I know how much you guys love those.
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