Have something between the factions that cannot be shared. Make it especially important to certain groups, ie to Forsaken to keep surviving as a race or to Night Elves to maintain Teldrassil’s blessing. Old grudges already exist and can be played off of. Both factions have sympathetic leaders and characters. At the end point, territory is lost and gained in exciting ways, ie Alliance gaining and healing Lordaeron, Horde gaining a full foothold in Kalimdor with all the resources they need - without necessarily pushing out NE. Losses for each faction should be significant but not comically evil like Taurajo on a larger scale. Unconditional surrender never needed to be in the cards beyond lip service.
That’s it. That’s all the foundation you need. Everything else is important but not entirely vital, for instance exploring the nature of each faction - the Horde’s relationship with honor despite its darker inclinations, the Alliance’s relationship with justice and its tolerance of the Horde’s evils under a gentler ruler.
A faction war expansion could be good. I was excited when I first heard about it. But it wasn’t about a faction war. It was about Sylvanas’ heinous, unspoken machinations and how she must be stopped, but one half of her opposition was ineffective and toothless while the other half sat twiddling their thumbs being directed at a far lesser evil that had no intention of wiping them out. Until patch 8.2.5 over a year later. And now a true faction war expansion will never happen.
I just peeked and my sub runs out in 42 minutes so I felt I just had to get this off my chest.