If the surface is indeed the same game world as BOTW1 then I hope that (A) substantial changes have been made to it to reflect how things have moved on since the defeat of Calamity Ganon, like new towns and such, as (B) that the sky islands offer a large amount of content to justify this feeling like a whole new game and not just an expansion pack.

I have a hunch that the surface world will 'merely' act as some sort of a hub between the islands in the sky and underground areas, if what we saw in the earlier trailer was indeed underground. So the surface I think is much smaller than in BotW and maybe even made up of corridors (at least in TP overworld style), with a lot of the regions from the first game inaccessible, but it is used to move up or down at places. If you think about it, if the islands rose up from the ground like we see Hyrule Castle do, then those areas don't exist anymore and are, on the surface, now just a big hole or something.

But as how it goes with Zelda theories, this is probably way off.

It will possibly be a case of the surface not only being a hub but also serving as a sandbox area for people to test the limits of the mechanics. If you look at ALBW the were changes to the map from ALTTP but it mainly served as a hub to the alternate realm I can see the same happening here only on a much bigger scale.

This way the surface is a familiar experience with changes which you return to every now and then like a world map/hub while the sky is the bulk of the new experience. 

Even after 4+ years I'm still finding out cool new stuff in BOTW1; some here I hadn't heard of for instance:

I really hope BOTW2 is equally as deep and provides new discoveries for years after its release.

i really hope they have under water exploration this time.

Yeah, I hope we get this too. I didn't feel it's absence was a problem in BOTW but it'd make for a cool addition.

Yes! I can totally see this being the case.

Managed to finally edit the post on the last page it had all this weird text so I just deleted it seems to have fixed the issue.

With the paraglider only allowing for, well, gliding, I wonder if you'll gain the means of actually flying at some point in order to travel more easily between the sky islands.

The glider may behave differently in the Sky compared to Ground.