There's been a lot of talk about scaffolding, but I've never said that they are in the game. What we've got now is up/down staircases, which do about what scaffolding would do without having that name, though they are subject to the same above/angle restrictions as other constructions. We can do more later.
A raw wall top... what is that? The top of a wall? It has a wall below it. A rooftop can have air below it -- map squares are divided into the upper and lower portions for the most part.
I think the main reason people were asking for rewall originally wasn't to do something from an angle above, but because they didn't like ugly ore walls and wanted to get rid of them.
Adjacent rewalls act like a continuous wall. The configuration that somebody pointed out from the mule video is still stable even with the cave-in code. Things like arches.
Water doesn't currently transfer temperatures. I was a bit worried about the CPU costs of the extra temperature updates, but I don't think it would be a big deal. Right now magma doesn't boil adjacent squares, or there'd be some troubles with its use. It does heat them to around 132F, or somewhere around there. Magma squares themselves are have magma temperature.
I'm not sure how I'd handle tents, but certainly any feasible material should be allowed over time to create partitions. Right now I think you're restricted to the same materials as bridges.
I think swimming becomes optional at 4/7, and required at 7/7 (it doesn't care about creature size at this point, I think, and that would be a tricky issue, especially if you fiddle with dwarf sizes, due to path-finding again). You can swim in a 1/7 if it is above a 7/7. When I say optional I don't mean to imply that there's an adventure mode interface for that option yet, but you can swim around in adventure mode if you get into the water, and I'm pretty sure you can jump in with the additional movement keys now.
A dwarf should swim if they have swimming skill. Otherwise, the dwarf will sink. I haven't handled density effects yet, so everything that isn't swimming sinks at this point.