As long as they don't make the caves too small or too dark, I'm personally fine with it. There could be sections where the caves are brightly lit up with glowing crystals, and spots where there are holes in the roof, letting sunlight dapple the cave floor. Maybe there are a race of people living...
I'm fine with it, as long as they include a fair amount of unexplored areas too. For example, the caves. There could be a whole other world under the land of Hyrule, just waiting to be explored. If Nintendo does decide to reuse BotW's landscape, they would need to have it dramatically changed...