What do you think of the idea of pipes? When placed adjacent to a water source block (and maybe it should work with a lava source block, too), it will transport the liquid through consecutive pipe sections until it reaches the end of the pipeline from which it will flow out. The fluid pouring out of the pipe will behave as if that last pipe section was a source block.
Perhaps there should be a limit to how long a pipeline can be. Or maybe you need a redstone-powered pump every so many pipe blocks to keep the liquid flowing. A redstone-powered valve is also a possibility. Maybe 90-degree angles and T junctions also.
If you support the idea, I'll leave it open for discussion the exact rules for how many pipe sections liquid will flow through, and what ingredients you need to build a pipe section, pump, and valve.
And if Mojang picks up on the idea, no doubt they'll have their own ideas for how to implement it, bleh.
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