It's what I'm hoping for, in all honesty. Zelda has a track record for doing the parallel world thing, and they could easily spin it so that we're seeing beneath Hyrule rather than an alternate version of it. I'd also like if whatever led us to explore underground has caused a moderate amount of topographical changes to the overworld. I've cited Tanagar Canyon and the Forgotten Temple in passing before, when chatting about BotW2, as an example of what I'd like to see more of as far as what is seen on the surface, that could potentially lead below ground. Hyrules lowest point in BotW contained a temple, so it only makes sense to me that it was intentially constructed below ground (and therefore forgotten until the canyon exposed it), so what's not to say we'd see more such temples in BotW2 if we were to explore beneath the surface?
The way they would go about connecting the above and below ground would be the key here though. If done well it'd be a great way for them to keep the world the same, while still having a significant proportion of it be new, and also have the existing world be altered in ways that would make the old fresh and still exciting to explore. BotW severely lacked underground sections and and caves at all, so I do hope what we saw in the trailer wasn't just like a very isolated thing. I would love to see more cryptic underground passages with the weight of existence on me the entire time.