TBF, the Dragon Isles were originally a raid. Nothing stops Blizzard from upscaling the concept from its original concept. Heck, they made the Broken Isles, Kul'Tiras, and Zandalar continent-sized areas. Doing the same for an area that was originally conceived to be a continent (their own words btw) shouldn't be a problem.
I disagree. We got quite a lot of the Azjol-Nerub theme during WotLK, and I highly doubt its something people want to return to anytime soon. An ancient race of spider-people really only goes one way, and it leads to something very similar to what we saw in WotLK and SHadowlands. An underground theme linked with technology opens up far more possibilities in terms of concept and potential that isn't hard-locked into a singular thematic. For example, you could have a zone with high rises and bustling streets, another zone that is a huge domed area surrounded by water, and another zone with an artificial habitat full of out of control mutated creatures (think Nagrand, but underground). You simply can't get that type of zone diversity from ruins run by ancient spider-people.
Well that entirely depends on how Blizzard presents the material. Unlike Azjol-Nerub, there are threats that can relate to an Undermine expansion. One such possibility is the mechanical threat highlighted in
The War. We've never faced a mechanical threat before, and comics and film have shown such threats to be substantial and entertaining.
So on one hand we have a monothematic spider empire that will do nothing but give people feelings of a rehash of WotlK's worst dungeons, and on the other we have a varied, cosmopolitan civilization that can offer us a threat we've never encountered before. This, along with a new class that offers a new thematic that isn't available in the class lineup.
If I were Blizzard, the choice would be obvious.