I've just started a megaproject of my own recently.
The idea revolves around a very large central dining hall. Think of a big + shape. With columns along the sides. Between the space in each column you could pack a set of seats and chairs that lead out to the middle, with plenty of space for walkways. Anyway, reaaaly big. Except also three floors high. So, digging out the same thing three times, then chanelling out the floor of the top two.
There is then a bunch of space avaliable in four corner sections, each three floors in height, which I will designate as residential. In anticipation of a dwarf economy, there will be there varieties, with the best at the top, with balconies, lots of rooms, etc, the cheapest at the bottom.
I'm digging this out at a depth where the ceiling is still a floor below the level of the local river, which of course I am having flow through a hole in the center, creating the waterfall.
Then, the water itself is going to pass through a grate in the floor of the hall, and into a tank beneath. This tank has four doors, n, e, s, w. Each leads to a room set aside for argriculture. So, i will rotate the flooding periodically, to keep everything moist enough to grow the shrooms, pig-tails, etc.
The end wall of each room, where the entrace is, has a line of floor grates, alowing excess water to be sluiced down into a tunnel below. This will then channel any excess water into the nearby bottomless cavern, hence removing the threat of flooding. :-)
The beauty of this, I feel, is that you have your massive dining hall enjoying the sight of a waterfall that itself helps produce the food on the tables. Furthermore, with the farms directly below, I can have an efficient set up in that respect, so the dwarfs don't need to walk far.
Further industry and mining can take place around this central residential/agricultural hub, in such a way that nobody lives too far from their place of work or other facilities.
I'm trying to dig this out right now with 22 dwarfs, 4 picks between em. Hoping the sherrif will keep patient about the lack of a personal room and office for a while yet.