Medivh felt that the mortals of Azeroth were at a disadvantage going up against the machinations of the Old Gods and their Void Lord masters, especially since they were able to see all possible futures and manipulate events at a cosmic scale to push the prime timeline towards a path that would spell the end of the universe as they knew it. Although the Burning Crusade had finally ended with most of the Legion's leadership killed (save Sargeras who was guarded and imprisoned by Illidan and the Pantheon) and their means for unlimited and unrestricted regeneration/reconstitution/resurrection gone with Argus' death, the demons were still a threat if reorganized by someone stepping into the power vacuum left by Sargeras.Medivh saw merit in Kairoz's goals but thought the idea could be improved and expanded upon without harming the fabric of the prime timeline. Medivh had already been in communication with Kairoz when he came across his astral spirit in the Twisting Nether and learned of the bronze dragon's intentions of creating an infinite army to combat the Burning Legion. Kairozdormu's astral spirit was bound to this shard, and it allowed him to peer into the infinite timeways without a restriction of a corporeal form. On Khadgar's possession was the shard of the Vision of Time Gul'dan possessed from Draenor, which coincidentally was the same shard that had impaled Kairozdormu by the hands of Garrosh Hellscream. Medivh explained to Azeroth's denizens that he had foreseen the true End Time of the universe (like Murozond had), and had been traversing the Twisting Nether and its connection to the infinite timeways, seeking a way to avert said crisis.
Accompanying Khadgar was his old mentor Medivh, who had returned from the Twisting Nether following his departure near the beginning of the Burning Legion's third invasion. Returning from the library of Karazhan, Khadgar greets the world's heroes and offers to provide a solution to heal the world-soul. Near the end of BfA's story, Azeroth's world-soul has been severely crippled between the damage inflicted by Sargeras' sword Gorribal and N'Zoth's consumption of her spirit energy to weaken/break his prison-bonds.
I provided a LOT of background, so bear with me. I figured the Lore forum would be the best place to share my ideas, and this post mostly focuses around the overarching story and not so much game systems. I'll also provide some Wowpedia links for reference. I don't have any skills with computer graphic art, so I don't have any pretty maps or concept art to show, but I'll throw in some related imagery of the mentioned story/content to give some visuals to break up the text. In the hope of breaking up some of the edge lord negativity on these forums, I made my idea for a "time travel" expansion that attempts to tie-up a lot of loose plot threads in Warcraft's lifespan into a cohesive story that takes place after the main events of Battle for Azeroth, and before what I imagined would likely be WoW's final expansion (dealing with the greatest cosmic threats like the Void Lords and a freed Sargeras).