We can build up, and crouch lets us build out, but we still cannot build down or underneath from above. I doubt hanging upside down from ledges will be added, so to fix the problem, I suggest the addition of a sledgehammer.
You hit a block with a hammer, the block moves away from the face you hit, if there is an empty space to move into. It can be exploited as an elevator going down, but that's a minor issue. I envision it letting me build underneath my skyway without having to build a ridiculous column from underneath. I would also use it to add an extra layer to other platforms from underneath, without working out from a cliff face. Being able to place a block on the level which you are standing, and hitting the block forward, rather than down, also allows a limited moving shield effect when in very hostile territory, such as the Nether.
This also allows hitting blocks upward from below. This is not particularly useful, since short columns of sand or dirt aren't hard to build and disassemble. Where it might be most useful is where a player placed, perhaps a roof, a space too low. Or, maybe a Nether portal is two by two, instead of two by three. Either way, it lets you move the blocks without breaking them down and placing them again. However, this aspect is only useful if hammering blocks into new positions takes less time than breaking them down and moving them. It should be balanced so hammering is easier for very short distances, as apposed to breaking objects down for long distance movement.
Perhaps hammering blocks would take a quarter the strokes to move a block as it does to pick a block apart. This could also lead to needing hammers of various materials for the various blocks, for efficient movement, thanks to better materials making for fewer strokes. Instead of the block not moving, or getting destroyed by using too low a hammer, it could take more hits to move the block. Maybe a steel sledgehammer would move an obsidian block one space in half as many strokes as it takes to break an obsidian block with a diamond pick, and a diamond sledgehammer would do it in a quarter the strokes. That would make short movements highly efficient with the sledgehammer, and long movements highly inefficient.
Over all, the sledgehammer would be correcting structural mistakes that would otherwise involve lots of, or expensive, deconstruction, or time consuming and dangerous framing.
You hit a block with a hammer, the block moves away from the face you hit, if there is an empty space to move into. It can be exploited as an elevator going down, but that's a minor issue. I envision it letting me build underneath my skyway without having to build a ridiculous column from underneath. I would also use it to add an extra layer to other platforms from underneath, without working out from a cliff face. Being able to place a block on the level which you are standing, and hitting the block forward, rather than down, also allows a limited moving shield effect when in very hostile territory, such as the Nether.
This also allows hitting blocks upward from below. This is not particularly useful, since short columns of sand or dirt aren't hard to build and disassemble. Where it might be most useful is where a player placed, perhaps a roof, a space too low. Or, maybe a Nether portal is two by two, instead of two by three. Either way, it lets you move the blocks without breaking them down and placing them again. However, this aspect is only useful if hammering blocks into new positions takes less time than breaking them down and moving them. It should be balanced so hammering is easier for very short distances, as apposed to breaking objects down for long distance movement.
Perhaps hammering blocks would take a quarter the strokes to move a block as it does to pick a block apart. This could also lead to needing hammers of various materials for the various blocks, for efficient movement, thanks to better materials making for fewer strokes. Instead of the block not moving, or getting destroyed by using too low a hammer, it could take more hits to move the block. Maybe a steel sledgehammer would move an obsidian block one space in half as many strokes as it takes to break an obsidian block with a diamond pick, and a diamond sledgehammer would do it in a quarter the strokes. That would make short movements highly efficient with the sledgehammer, and long movements highly inefficient.
Over all, the sledgehammer would be correcting structural mistakes that would otherwise involve lots of, or expensive, deconstruction, or time consuming and dangerous framing.