Green roofs have a similar effect but it not as strong. Also, on the fish thing, I think it's called aquaponics(Aquaria+ hydroponics).
Nitpick, though I can see someone coining and using aquaponics for their business, hydro- is the greek root for water (with -ponics meaning work or labor), aqua- is latin. Mixing the two makes me cringe. I've heard it called aquaculture, or just fish farming.
What I'd love to see is more subterranean apartments and other buildings (earthscrapers) so that we can use existing skyscrapers for more sensible things like wind/solar platforms, epic gondola landing-pads, and the farming thing maybe. In an extensive underground city, you could tear up most roads on the surface and rely on tunnels and lifts, and use the huge amounts of space you gain for parks, farms, whatever you like. I doubt the engineering challenges are worse, just different - ventilation, combating shallow water tables and poor soil support, etc., fires (possibly), earthquakes (probably), and bioterrorism (what else is new?) are its biggest problems, but it'll get easier and cheaper as more people do it, as with everything. Maybe a pipe-dream, maybe optimistic in that people would still want to be surface-dwellers and would probably stay just as crowded on the surface, but I guess DF rubbed off on me for wanting this.
Edit: This is what I get for posting to nitpick, everything I just said has been said here by others. Durp. Sorry gaise.