From a narrative standpoint, I think the fundamental idea of the First Ones is to reintroduce what they lost when they decided to make the Titans and their Cosmic equivalents active players in the story.
In the early days, the Titans were essentially presented as the primal creator deities of the WoW universe. They were mysterious, had long since disappeared, and the remnants of their culture was scattered around Azeroth and other worlds we discovered. A single living Titan remained, and he was framed as the archvillain of the series.
After defeating the Old Gods and with Sargeras and the Legion next up on the chopping block, they needed new avenues to explore to create cosmic-scale conflicts, so they started planting the seeds for what became the war between the six cosmic forces. Which I actually think is pretty cool, FWIW.
They lightly retconned Chronicle as being written in-universe from the Titan perspective rather than an infallible developer bible, and have introduced further unreliable narrator elements with Grimoire offering opposing interpretations while not committing to either interpretation being correct, introducing a new higher level pantheon of creator deities.
The First Ones are essentially filling the role that the Titans originally did: mysterious ancient creator deities that are long gone, with facilities and relics left behind on the worlds we visit. I'm gonna go ahead and call it now: "Zereth ____" is going to be the new "Uld____." There will be a Zereth facility on every cosmic plane we explore, inhabited by automa. Zereth Vitae, Zereth Lucis, etc.
There's probably going to be six of them, one for each of the cosmic forces. Most or all of them will be dead, but there might be a final one still living that will be the new archvillain of the universe, maybe the creator of the Void or even the Fel (consider that we don't have a proper Fel pantheon, only Sargeras the Fallen Titan of Order).