Ok first off, waterfalls are already implemented. It acts similar to sand, as it is immobile until you affect it, causing it to fall and act like normal water again. So affecting them destroys the waterfall permanently. Secondly, I support waterfalls become a common feature in biomes, and id like to have placeable water "blocks" in order to recreate the phenomenon for my own purposes.
If fluid pipes and pumps are implemented, then this would be quite simple to do, pump water from the lower pond, into the upper one, where it overflows and falls down. An infinite source of water for natural occurrences could easily be explained by there simply being a background river. Or, if you don't mind "cheating," the game could spawn a pump and pipe setup to do this. Perhaps make them invisible or something so that it doesn't break immersion.