During the Fourth War, the Battle for Stromgarde and the Battle for Darkshore, there were rumors that a Barrens Warfront (likely involving the tauren) or Quel'Thalas one (likely involving the blood, high and void elves, possibly a final confrontation between all three Windrunner sisters at Silvermoon) were next, as two Warfronts seemed somehow incomplete in the storyline.
As it stands today, we have literally no idea on the state of Kalimdor, for example, the exact political status of Ashenvale, Feralas, Mount Hyjal, Moonglade, etc. (the night elves are not allowing the Horde into their lands - but what constitutes their lands exactly after the Fourth War, we still have no clarification).
This could provide an interesting scenario - as it would feature an updated Broken Isles, and show the political status of the different areas there - AND explore the very complicated relationship between the night elves and their nightborne cousins, which was mostly avoided in the game. We forget that both Suramar and Highmountain are extremely close to both Kul Tiras AND Zandalar, both occupying critical strategic positions, and Azshara must have taken the Tidestone from the Tomb there during the war anyway.
On the Alliance side:
The night elves, led by Tyrande Whisperwind and Malfurion Stormrage after their victory at Darkshore, lead the kaldorei forces from Kalimdor and take their navy to the Broken Isles, angered that Thalyssra and the nightborne refused to leave the Horde, and still supported Sylvanas during the conflict after the Burning of Teldrassil. Now I honestly have no idea whether Azsuna and Val'sharah have joined the night elves or not, but there seem to be some hints in-game that they have taken sides at least marginally (for example, the night elves of Val'sharah have seemingly established close trade links with Bradensbrook in one Alliance-only quest).
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The night elves, their military predominantly led by Maiev and Jarod, two other natives of Suramar City familiar with their ancient territory, lay siege to Suramar, seeking to "reclaim" the ancient settlement for their people. They try to surround and blockade Suramar on both sides, Maiev sending forces from the Isle of the Watchers (and possibly Nar'thalas) in Azsuna, and Jarod sending forces from the Black Rook Hold and Bradensbrook in Val'sharah. I'm not sure about the Moon Guard Stronghold, or whether Farodin will make any appearance, but both would be very interesting.
The second Siege of Suramar begins - Tyrande this time leading the siege once again, but staunchly opposing Thalyssra, rather than fighting alongside her, calling upon Elune's wrath as she faces Thalyssra and other nightborne leaders in open battle.
On the Horde side:
Thalyssra immediately calls for backup after she realizes Tyrande has returned to Suramar to attack her people. Due to Oculeth's advanced telemancy network, the nightborne are able to transport most of Highmountain's military to Suramar very quickly, and Mayla herself intervenes on their behalf at the front of their army. Silgryn and Victoire lead the Duskwatch in defense of their ancient city, possibly facing off against Jarod, former Captain of Suramar's Guard, and someone whom they might remember from their pre-Sundering days, and his sister Maiev and her Wardens - a very interesting scenario, with the Wardens fiercely fighting the spell-fencers. Valtrois tries to bolster their magical defenses by drawing from the regional ley-lines, and she and Thalyssra activates their nightborne constructs and unleash their captured animals, to fight the night elves and their nightsabers, hippogryphs and ancients.
As the Kul Tirans come to assist the night elves from neighboring Boralas, Oculeth and the other nightborne magi manage to bring in some of the Zandalari forces to even the odds against the invaders.
So in the end, the Battle for Suramar would = the night elves and the Kul Tirans for the Alliance VS. the nightborne, Highmountain tauren, and the Zandalari trolls for the Horde.
Ideally, I think the nightborne would actually WIN this warfront - it is only fair to the Horde, as the Alliance won both Stromgarde and Darkshore - but very narrowly. Perhaps in desperation, as Tyrande and the night elves begin to break through Suramar's defenses, the nightborne would try to construct a mana bomb against the kaldorei invaders, or perhaps use their temporal powers to somehow eliminate Tyrande's military - and Tyrande manages to barely stop them, but is too weary and depleted afterwards, falling unconscious to the ground after she saves her people, causing the night elves and Kul Tirans to finally retreat after a long and very intense battle with massive losses on both sides.
In the aftermath, the mutual cooperation between Suramar and Highmountain and even the Zandalari is greatly strengthened, with all three races contributing to strengthening the city's protection against future Alliance invaders. While the frustrated night elves realize that Suramar is off-limits to them, and perhaps instead refocus on settling Azsuna and Val'sharah with Kul Tiran assistance, leading to a new if very uneasy status quo in the Broken Isles.