Valley Biome generation

Since we're getting a mountain update soon, I think that it'd be nice to add a "Valley Biome" of sorts. Unlike the rivers that cross mountains we have now and unlike Plains, which are mostly flat, we'd have a biome that slopes downwards, many times beneath the sea level (something we don't really have yet, unless you count rivers). These wouldn't just be Plains biomes surrounded by mountains; the terrain would slope down from mountains to a lower point, getting an increasingly gentler slope. At its lowest point, we could have a river generate sometimes - perhaps even lakes. These biomes would preferably generate away from oceans, but if they did generate close to them, it could be nice to build dams between them. Perhaps forests (mainly oak and birch forests, maybe Taigas, too) could have Valley variations akin to Hills. Most forests in minecraft are flat or in hills, so it'd be nice to have a variation that slopes downwards. Maybe valleys could be just that - a downwards sloping terrain generation of other biomes, with the occasional river down the middle.