So basically playerbase came to conclusion that Dragon Isles is the new continent - simply because we're out of options now. Sure there's Undermine, but srsly, a 1 goblin-themed island as an expansion continent? That's just ridiculous. Dragon Isles seem more likely since we were promised a homecoming in 10.0 so we rule out new 'realms' such as Shadowlands for the next expansion. At maybe one exception...
To sum it up - everyone believes in Dragon Isles because everything else in terms of new landmass is ruled out. The only question is - will the story itself also be limited to the confines of the new continent or is it wider and the landmass is at best a source of secondary quests, not the main plot.
The examples of these 2 approaches are respectively Shadowlands and Legion.
Shadowlands offer us an adventure where each and every event is connected to the local problems and local villains native to the new continent. Big story is the local story at the time. Sure Jailer's actions potentially affect Azeroth but he never ever sets his foot into mortal plane and Azeroth specifically, at best trying to demolish it from a dimension-far distance.
In case Dragon Isles goes this way we may not expect to see massive light vs void war but threats connected to... well, dragons seemingly. Some evil dragonflights who are bigger, stronger and arrogant, dragon empire from nowhere etc. To me that seems just overblown, dragons only can hardly be such a big problem. Only way possible that I see for that if they link it to the Lifelands and Elune somehow. "The vassal of life masks treachery, beware the eyes of green" or something. So perhaps one of the Lifelands' pantheon members, lets call him 'Agressive Dragon Lord', decides to claim Azeroth and boost some new 'Dragon Emperor' of a kind, and Elune will try to help us out with this. Also that might bring us back the story of Botani and Overgrowth and remind that life can be evil too sometimes. That is... well, maybe somewhat fresh. I also expect an endgame zone in Lifelands in this case - exactly like Argus in Legion. Elune Realm, Dragon Lord Realm, some gardens in between, all in 1 patch so that players are not bored being far from home again for too long. We help saving Azeroth from massive dragon invasion mixed with Draenor-style Overgrowth stuff.
In case its Legion's model, the Dragon Isles will play same role as Broken Isles - just some new place that has to be there. We do local problems in questing, have a raid or two similar to Emerald Nightmare and ToV, and then turn to the actual problem that is bigger than the isles and shows itself over all Azeroth. In this case, I'd say, it is said Void and Light. Why? Well because we have a lot of story development leading to it. Azshara's story in BfA mimics that of Gul'dan
in WoD, Xal'atath is around. Letters from Scarlet Crusade against Anduin and Calia are also clearly a buildup, as well as 'golden one claims vacant throne', namely Turalyon. Yrel is also tied there since BfA. So this is clearly a preparation for light and void clash. In this case the "Argus" of 10.3 will be most likely the void realm.
The only question for me here is - will they combine dragons with light and void or not? Do they feel dragons can be an expansion on their own or Dragon Isles is just new Broken Isles, a stage for a greater conflict?
Sure, dragons and all light\void stuff don't seem like a single thematic stuff, but BI are also not all about legion except Broken Shore and the Tomb. So perhaps the squid-like temple on the DI is a new void-themed Broken Shore and\or Suramar while rest of the isles have their on local problems.
On the other hand, if dragons are indeed a separate expansion, what is the continent for Light\Void? Revamped old zones? Realms-walking again, like SL? I don't think they'll risk making an entire expansion placed in cosmic realms again after SL. Anyway, what do you people think?