I am thinking about at least three possibilities for BG3 - 1) Irenicus' return 2) Bhaal's rebirth 3) Rise of new powerful Bhaalspawn or a rise of new god, such as Bane, or Mystra, or rise of multiple gods and they battle each other.
I will write 1) right now as I am tired
1) Irenicus' return (the time actually takes place after SOA)
As Irenicus fell into the abyss with many demons holding his body, it was clear he would burn forever in eternal hell. Many thought Irenicus' life ended there, however Irenicus was even ready for the final moment in hell.
As he was falling into the abyss, he gathered his final magic strength, combined with a segment of divine power he obtained from Tree of Life, although everyone thought his power from Tree of Life was destroyed after he lost battle against heroic Bhaalspawn (You). With that power, he was able to defeat those demons and he was able to fly for few minutes in order to reach the safe cliff. Now one last time, he started meditating, desparately searching for the Bhaal's essence in him, which is part of your soul. When you defeated Irenicus, you thought you had got your soul back. However, a segment of divine (Bhaal)'s soul was missing. That soul, was still in Irenicus' hand.
With the deep meditation, Irenicus started to make incantation. The demons caught Irenicus in sight and they were approaching. Irenicus was smiling and he transported out of the abyss, but to the other abyss, which is safe for Irenicus. This new ability called pocketplane, is supposed to be used by Bhaalspawns only, however Irenicus was somehow able to figure this out within a short-time. Such a genius vile mage. In this new abyss, or his own plane, there was nobody but Irenicus, and from there, Irenicus could enter any realms he wanted to. There is an imp that serves Irenicus, a Bhaal's creation who serves any of Bhaal's children.
Irenicus, of course, not going to the hell again. Even if he is sent to hell again, he can use pocketplane any time to his own plane. Irenicus wants to revenge the demons and those heros who took his life twice, from Suldanesenar and even from the hell, but to do so, he still needs to be very strong, and as he always dreamed, he want to be a god himself. With the segment soul of Bhaal and the ability of pocketplane, Irenicus once again try to acquire the divine power from the Tree of Life.
Bhaalspawns are still battling each other, and no one knows if this evil mage is ever coming back. The hero just defeated Yaga-Shura, the fire-giant Bhaalspawn. Irenicus will take this as advantage. He will kill the last Bhaalspawn and be the Lord of Murder himself. He will simply wait until Bhaalspawn kill each other so he can only fight the last one. The last Bhaalspawn will be the toughest one, Irenicus expects. To prepare the battle, he trying to be a godling, or to be close to god as much as he can. He hires those parasites again, and gathers drows as he did before, but this time, Irenicus allies were many. He was able to use propaganda on Bhaalspawn's battle, and his political strategy started to work.
The governments of Athkatla, they were losing trust from people. Irenicus was becoming a new hero. He even guranteed people that people will live longer and stronger if they can get the secret elements from the elves. Many men were of course tempted. It was out of control and it was very clear that elves will have a war against humen soon. The people in Athkatla separated into two factions, one for pro-elves, the other for hunters of elves.
One of the mysterious 6 robed wizards in the circular table now revealed themselves. The other 5 still stays in mystery. His name is Hector. His secret armies had been growing at the unknown island of the western ocean of Baldur's Gate. Now Irenicus allies with Hector, but it isn't sure how long their alliance will even last. But Irenicus concerns none, since he has other alliances and his power is much greater. The one last Bhaalspawn, will have to fight Irenicus once more, to get the last piece of divine soul back. But it won't be easy, Irenicus is now a new god with the divine power he obtained from tree of life. He even defeated the Leaf Lord, the semi-god guardian of Tree of Life.
Irenicus started to hunt other gods.. until he becomes the ultimate god, AO, himself.
AO- one who created Bhaal, Mystra, Bane