It was basically a placeholder biome, similarly to how the "Deep North" was nothing but icebergs, and the Ashlands was nothing but Surtlings. They were, however, intended to be used for later.

That said, the Mistlands is becoming the next biome installment to the game. The next major update will include it, hopefully in its entirety. I say that because, also, at some point they're adding cave-type dungeons to the Mountain biome. I'm unsure if it will be part of the Mistlands update, or a separate one.

So anyway, once Mistlands releases fully, Plains won't be the endgame zone any longer. ;)