This is going to be even more speculation-heavy than most of my lore posts, because even with Chronicle, we really don't get a lot of detail on what the Titans were like before they were, well, Titans.
One of, if not the biggest reveal in Chronicle was that all of the Titans in the Warcraft universe were once something called World Souls, and that Azeroth is one of these future Titans. These World Souls develop over a long period of time and, until relatively recently (in the cosmic timeline,) they had help.
According to Chronicle, Aman'thul was the leader of the Pantheon partially because he was also the first Titan to emerge. He traveled the universe and eventually discovered another planet that had a similar World Soul within it. He cultivated life on that planet and prepared the world for its metamorphosis into a Titan.
Over time, the Titans grew in number, with Aman'thul joined by Eonar, Khaz'goroth, Golganneth, Aggramar, Norgannon, and good old Sargeras.
The seven Titans went around to every world and did the same basic thing - they would cultivate life and ensure that the planet was healthy and in good order, and then they would wait and see if the planet ever became a Titan. Not every planet did so - most planets were just your normal hunks of rock floating in space, but the Titans were fine with creating or cultivating life on these worlds - they were happy to see the world filled with life. It's likely that Draenor was such a world, and also Argus.
In fact, there's an interesting thing to note about Draenor. While the world is considered, generally, to be more "savage" than Azeroth, the elementals are far more harmonious and cooperative. The reason for this is that there is a fifth element called Spirit (or Chi if you're Pandaren) that kind of catalyzes friendly interactions between the elements. However, this is also apparently used by Titans to grow, and because Azeroth is destined to be such a powerful Titan, she drew in the vast majority of the Spirit, thus leaving the elementals in total chaos. The lack of spirit may actually be the only reason the Titans even knew there was a World Soul inside of the planet - in usual cases, they just sort of act as if there is one just in case.
So here's what I wonder about:
The Titans created sapient life on these planets - certainly in the case of Azeroth, but also probably on other worlds (the Gronn and Botani on Draenor are probably both Titan creations.) It stands to reason that there were intelligent creatures living on the planets that the Titans emerged from.
So what happened to those planets? And what happened to those people?
We generally see the Titans as humanoid, even though they are so vast that a humanoid shape doesn't even really make sense (consider that we have feet and legs because we generally have a ground to stand on - that's harder when you're so big that outer space is basically the only place you can fit.) It's possible that this is really a kind of visual metaphor - we think of the Titans as looking humanoid because it makes it easier to think of them as intelligent beings that way.
So when Aman'thul reached down and plucked Y'shaarj out of the surface of Azeroth, was there really a human-looking hand, or was it something a lot harder to comprehend?
It wouldn't shock me to find out that the Titans actually sort of stay planets - that we're really talking about these big spherical balls that are going around in space. But I'm also not sure if that's what they're really going for.
Really, the question is: is each World Soul a planet, or is it simply inside a planet? When Aman'thul emerged, did the planet he had been in change shape, turning into a massive humanoid? Did that planet experience a massive apocalyptic destruction as the Titan climbed out? Or is there still a planet Aman'thul out there that just happens to be empty of its World Soul but is otherwise fine (setting aside the likely possibility that such a planet would be one of Sargeras' first stops on the Burning Crusade.)
And if there were people on that planet, what happens to them?
The revelation that Azeroth is a Titan does really raise the stakes in an interesting way. We certainly want to prevent her from being destroyed by Sargeras (assuming that's actually what he wants to do, and there's some serious reason to doubt that) or corrupted by the Old Gods. But what about the Titans' plan? Is that something that we would actually want to happen?
Azeroth is either going to emerge very soon, or it will be some event that is referred to but doesn't happen as long as the game is still around. But when it does happen (if it does happen,) are the people on Azeroth going to be ok?
There are two scenarios - one is that the planet cracks like an egg and the Titan steps out, leaving everyone on the planet either dead or living in an Outland-like ruin of a world. The other is that the Titan sort of spiritually leaves the planet - there's still a physical world there that is unharmed, but her power has completely left it. That is probably the best one could wish for, but it does raise some interesting questions about just what Azeroth would be like without the Titan inside it.
Of course, I suppose there's a third option - that Titans never really leave their planets, and that instead the planets just become Titans. There is a description of Sargeras in which his skin is described as being something like a landscape (that is now erupting with Fel Volcanos.)
So will we instead start to see maps of Azeroth where the Eastern Kingdoms are just below the collarbone? Will sailors need to be careful to navigate around her arms? This absurdity is a big reason why I suspect the humanoid shapes are more metaphorical.
But the other question this raises is: what happened to the people living on the other Titans? If we assume that the birth of a Titan is not a horrifying apocalypse for anyone living on that world, does that mean that while Aggramar was visiting Sargeras to try and talk him into listening to him about Azeroth, there were a huge number of people witnessing that event and were also snuffed out when Sargeras killed Aggramar?
I honestly don't think we'll see this for ourselves - I would guess that while Azeroth might be conscious and intelligent even before she "hatches" (a lot like Wrathion was,) she probably has a remaining gestation period in the millions of years range.
But given that there are all these world Sargeras is destroying, with vast populations upon them, perhaps his turn against the Pantheon was more than the death of several god-like beings, but also entire species, cultures, and histories.