If we are getting an Old One expansion I hope N'zoth manages to release/awaken both Yogg-Saron and C'thun. If my lore is correct they are not exactly dead even though we "killed" a weakened version of them in previously raids, they just got pulled back into their prisons again.
In Azeroth's nascent state they cannot be killed because they have burrowed themselves so deep into the core of the planet that removing them would do immense damage to our titan. When Aman'thul shredded Y'shaarj it created a permanent rift in the planet pouring out arcane energy which eventually became the well of eternity. For that whole reason the Titanforged was created to subdue and imprison the rest of the Old Gods to remove the threat and still keep the world soul intact.
I believe our titan is on the verge of waking up, never in the history of Warcraft has she been more active communicating and speaking through Magni. The previous titans sensed she might become the strongest of them all, and I originally thought she would be needed to defeat Sargeras, but it looks like she might become our greatest shield against the void instead. But if Azeroth awakens, maybe in a weakened state because of all the corruption she had to endure having the planet completely engulfed in old ones corruption for so long during the time of the Black Empire, it will give us free reign to finally go after the last living old gods and purge them without having to worry about further damaging our titan.
Yogg-Saron is my favorite Old God. I love his design, the voice acting and all the lore surrounding Ulduar. After Y'shaasj he was the strongest old god and the last one to fall against the titanforged. I would really love to see them all return in their original form after being freed from N'zoth somehow. They were extremely powerful back in the day, as high as mountains and only looking at them was enough to make you go mad.
I for one am glad we're wrapping up the story of the Burning Legion after nearly two decades. The void is going to be an interesting new chapter.