A Light holy war expansion would be a bad idea for several reasons, including;
The Light is still better/less bad and not as much of a threat as the Old Gods, Burning Legion or the Jailer. Naaru don’t want to destroy the universe and genocide everyone, not even everyone’s “favorite” naaru. Speaking of her, Xe’ra attempting to imprison one elf and forcibly Lightforge another pales by orders of magnitude compared to the laundry list of atrocities the Old Gods, Sargeras and the Jailer have, plus the naaru are kinder to their followers than the aforementioned three. Even the Titans have far, far more blood on their hands than the Naaru (remember Algalon saying he’d killed “a million million lives” – that’s ONE TRILLION PEOPLE – for not fitting the Titans’ vision).
Making the Light less benevolent required retcons, and people are starting to get fed up with the writers’ incessant retcon habit.
Whatever flaws got retconned into them, the Light, Turalyon and AU Draenei have good qualities and legitimate grievances (Burning Legion, Iron Horde, Old Gods). For all that some fans talk about nuance and corrupt factions, Blizzard seems to be retconning and railroading them as moustache-twirling fanatics despite that and railroading us to fight them.
The “extreme order is equally bad as extreme chaos” shtick has been cliché for decades. It’s not even the first time Blizzard pushed that story, even in Warcraft. They were setting up the Titans for that with Algalon and the Mogu from Wrath to MoP, and in other Blizzard games we have Malthael’s Reapers (Diablo), the Vishkar Corporation (Overwatch) and the Tal’darim Protoss (Starcraft) to name a few.
A better solution would be a Light/Void showdown where we can choose a side or resolve their eternal conflict, temporarily or maybe permanently. If they want to keep going the “cosmic war” route, they can, and we can flesh out their differences rather than yet another “they’re as bad as each other at the extremes” story arc.
Also, an expansion where the Alliance are the aggressors could be its own thing without the hackneyed “holy war” angle.