It's been done. 8.x was the south seas, naga and void expansion all rolled into one. I know you expect the void lords to be an expansion of it's own, but my suspicion is that it won't be a void lord expansion but instead a Light expansion.
Yep, you heard right, for a while now void users have been getting a good side aspect to them. We have some ethereals, all the void elves, and ofc the shadowmoon clan in the Mag'har orcs. Forsaken priests too - then combine that with the light now showing a supposedly evil side with Xe'ra's treatment of Illidan, then the revelation of the Lightbound.
Think, they've done void a few times, especially with N'zoth - it's not going to happen, blizzard love cheap twists, and it would be so typical thar the light is going to be challenged, and we have to come against something that's gone wrong with the light. Maybe Shadowlands was the light's way of dealing with people and death, but the vents with the Jailor and the maw has necessitated some of its agents we shall meet to take action and need to be stopped.
Instead of void themes, you get light themes to deal with . Yes the void races (void elves, mag'har orc shadowmoon, Ethereals) play a large role, and yo have to deal with some void lords, because part of the reason the light's gone crazy is to stop the void lords, but the expansion will take a twist and be about the Light.
So... would you have preferred BFA to have been centred around Azshara and the naga, with a much bigger Naz'jatar being the expansion zones, with Kul'tiras and Zuldazar coming in 8.2 (like we had Nazj/Mecha - and a much smaller faction war focus), then 8.3 being N'zoth.
They did the same with the legion, rather than deal with it in 2 expansion, we went Argus and finished it off. No more legion. Likewise I expect no more voit