Alluvial

"An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time."

Much like plains/Savannah, a mostly flat and charscteristic-less plate of land, but in this situation the block pallets would be clay, coarse dirt, dirt + roots and something like a mix between wet farmland and path block but full size instead of transparent. 

In the midst (randomly) of the alluvial plain should lay a river or a small/medium marsh (floodplain). Within this concept I would specifically imagine a weather related timing to when the floodplain will fill up and become a regular river again for X amount of time. It starts raining and if it persists for longer than that areas "threshold" for flooding to occur, the floodplain will fill. 

Could make for interesting situations and interaction from the world unto the player. 

As far as I know we don't have a biome that can directly threaten the player and since alluvial plains rely on sedimentary deposit, some cool lore could be woven to it. 

Sorry for formatting. On mobile and rambling the idea as I come with it