While it’s clear that Blizzard is eager not to spoil the future of Battle for Azeroth, it seems pretty unlikely that players who want to be pure evil will ever get their day to shine in World of Warcraft. Despite the decision being added into the game, the good guys will likely win and the threats to Azeroth will continue to roll on by, getting knocked out by players one at a time.
In Kul Tiras, the Alliance's initial attempt to gain the nation's assistance ends disastrously when Katherine Proudmoore, Lord Admiral of Kul Tiras and mother to Archmage Jaina Proudmoore, not only refuses the Alliance's request for aide but also orders Jaina arrested and exiled for her role in the death of her father, Daelin Proudmoore, in the aftermath of the Third War. It quickly becomes apparent that Kul Tiras has become mired in corruption and outside threats, so Anduin and Genn Greymane send Alliance Champions to aid the Kul Tirans and earn their trust. Aiding them in this quest are Flynn Fairwind, a "retired" privateer, and Taelia Fordragon, the idealistic daughter of Bolvar Fordragon, former High Lord of Stormwind and current Lich King of the Undead Scourge. The Alliance is successfully able to earn the trust of Kul Tiras by combating the pirates of the traitorous Ashvane Trading Company in Tirisgrade Sounds, the Naga and corrupted agents of Queen Azshara in Stormsong Valley, and the occult forces of the ancient, ghostly warlord Gorak Tul in Drustvar. After these threats are defeated, Katherine is left despondent at her inability to lead her people, and resolves to help the Alliance find Jaina and reconcile with her. The Alliance eventually track Jaina to the prison island of Fate's End, where Gorak Tul, having survived the events of Drustvar, has kidnapped her and is tormenting her with visions of her past failures. The Alliance champions and Katherine destroy Gorak Tul once and for all and rescue Jaina, who is finally able to let go of the hate, anger and obsession with the past that defined her ever since the destruction of Theramore. The Alliance and Proudmoores then unite to defend the capitol city of Boralus from a massive pirate siege, after which Jaina is named Lord Admiral and pledges Kul Tiras' navy to the Alliance.
The recruit also grants the recruiter levels, with the latter gaining one level for every two levels the recruit gains. There are a lot of rules and restrictions to that, however. It all basically boils down to leveling alternate characters together. The recruiter can even earn epic mounts and battle pets if you stick with the game for a couple months.
At the end of Legion, the titan Sargeras was imprisoned, but not before he plunged his sword into the planet Azeroth. This not only devastated a massive area (much of the desert zone of Silithus is now cracked and scorched), but badly wounded the gestating titan inside. While the heroes managed to ablate the worst of the damage (at the cost of the majority of their artifact weapons' mythic power), the world is still wounded and bleeding a substance called "Azerite," which has great magical potential. The Horde Warchief, Sylvanas Windrunner, attempts to consolidate Horde power on Kalimdor and gain a monopoly on Azerite (which is primarily found at the southern end of the continent). Her campaign to do so, the War of Thorns, formed a pre-launch event for the expansion and ends with the major Night Elf holdings on the continent seized or (in the case of their capital city of Darnassus) annihilated. The Alliance makes a retaliatory strike against her home base, the Undercity, which was formerly the human capital of Lordaeron. This succeeds in driving the Horde out, but before leaving Sylvanas saturates the area with disease and toxins that render it uninhabitable. With this tit-for-tat military exchange, the Horde has almost complete control over Kalimdor, while the Alliance has near-total dominion over the Eastern Kingdoms. With further conflict inevitable, Battle for Azeroth takes the two factions to the continents of Kul Tiras and Zandalar to recruit new allies in order to turn the tides of war.