All your ideas are good… and there’s even more that could’ve been done with Dragon Isles scenario after Legion. I’ve had a similar scenario in mind but different for a Dragon Isles expansion, one less focused on the faction conflict.
Remember how in Legion all dragons were put on the extinction clock after defeating Deathwing because that made them sterile somehow? That quest “The Last of the Last” with Senegos and the blue dragons. A Dragon Isles expansion would be the perfect time to address that issue. Not to mention Alexstraza, being compassionate and the Lifebinder, would want to get right on saving her people from extinction. This could even address the issue of Sargeras’ sword stuck in Silithus by having the Chamber of the Heart physically located on the Dragon Isles.
We even have dragon characters with closer ties to each faction who can act as guides an emissaries; Chromie is a bit closer to the Alliance, I think, and as of Legion, we have Ebonhorn for the Horde. Most dragons are neutral and happy to work with both sides. In fact, this could even be a good time to bring in Sabellian and the Black/Nether dragons from Outland.
While the faction conflict would take a backseat, there’s still opportunities to explore that. Genn’s out for Sylvanas’ head, Anduin wants answers if not bloodshed, Tyrande and Malfurion wanting to help the green dragons to honor Ysera’s memory (Tyrande being almost as anti-Horde as Genn). On the Horde’s side they could bring the Nightborne to study the Titans’ power and technology stored on the Dragon Isles and Sylvanas wanting to weaponize that against the Alliance while also getting back at Genn for ruining her Val’kyr plan. Not to mention given her increasing acceptance of using undead, I’m sure Sylvanas would be willing to make a lot of undead dragons given the “possible dragon extinction” element of the plot.
Plus there’s a great major villain who’s ideal for the theme of the Dragon Isles. Chromantus. The only successfully created Chromantic dragon from the books; Warcraft’s five-headed answer to Tiamat from DnD and King Ghidorah from the monster movies. He’s practically unkillable; it took four aspects’ full might plus Thrall filling in for the Earth Warder/Deathwing just to incapacitate him so that’s a majorly tough raid boss, Chromantus can use the powers of all five dragonflights AND he’s currently locked in a Blue dragonflight vault where enough arcane magic - we’re talking from the world and its arcane nexus - will wake him up again…
Which brings me to another major villain for this hypothetical expansion; Queen Azshara. At this point she’s still gathering power and ostensibly working for N’Zoth, so she would be very happy to get her hands on the Titans’ power stored in the Dragon Isles. Also, remember how Chromantus needs arcane magic to wake him up? If any one person has enough arcane magic to wake Chromantus up, it’s Azshara.
Chromantus and Azshara also have shared ties to the Void AND Azshara’s got the cleverness to bargain with or manipulate Chromantus. She could simply get an alliance by promising him the chance to get revenge on the Aspects. Azshara could start implementing her plans for N’zoth or even try to harness the power of the Titans for her own ends (it could even provide the possible apotheosis plan she hinted at in Nya’lotha).
As for Chromantus, he could try to sway other dragons to his side by promising a cure for their sterility (whether he actually can fix that or not is irrelevant for persuading them; villains often make false promises in fiction). That creates conflicts of interest and doubt, and adds some “morally grey” such as dragons considering whether or not working with Chromantus or fully joining him might save them from extinction. We might not even have to resolves the issue of the dragons’ sterility in this expansion, but by expansions’ end, a solution will be found, if not implemented.
Just sharing a similar idea for how a Dragon Isles scenario could work. And I think both mine and your ideas, off the top of your head and a few minutes of brainstorming from me, are superior to what we got in BfA and SL.