Currently the endgame evolves around the three "pillars", rated pvp, m+ and raids, all group content. One distinct feature of this system is that you mostly progress your character for a pillar by playing that pillar (it's not always successful in that regard, but players always dislike that). Blizzard has been experimenting with implementing roguelike play with Torghast, and it hasn't been that succesful. But I think that's mostly due to two reasons, a) it's not a separate pillar, but instead a roadblock you need to pass to access the three real pillars ("choreghast") and b) class balance is rather nonexistent.
Could the next Torghast be a viable fourth solo pillar if it fully gained its own internal reward structure? Ie you don't have to run alts through if you don't want to, and it provided at least half-decent loot (possibly through the vault)?