I don't doubt that some concepts surrounding BotW's world design may have been considered more deeply than we saw in the finished product, but that's common for any game. Like when you flesh out a world, you really have the opportunity to consider why it is the way it is, but this may never actually be explained for added mystery and whatnot. But because they likely didn't use every idea they had for BotW, in that sense, sure, they had some unused plans that could probably connect to some open ends they left in BotW--like the Zonai--and have that work as a transition into a sequel. But no, I don't think they actually planned to make a sequel. They simply had some ideas that they thought they could bring to life in DLC and then decided that it was substantial enough to make its own game.