I’ve always thought that the struggles of a new FR deity could be pretty interesting. Think about what the Bhaalspawn would be up against: 1) few or no followers in a setting where a god requires worship to survive, 2) no divine portfolio to speak of, and 3) a fortress / headquarters that is still floating somewhere in the Abyss – not friendly territory, even for an evil PC.
Under those circumstances, I think a divine campaign would be a battle for survival. It’s suggested at the end of ToB that Cyric and his allies will be coming after you (whether the player is evil or not) because Cyric took over Bhaal’s divine portfolio. The player, as a nascent god, would be facing off against some powerful deities.
Terrible odds? Yes. But that’s great for a story hook.
Initially the player may just be staving off imminent disaster as Cyric and his friends move quickly against the player – several strong deities against a relatively weak one. I could imagine the player being forced to abandon the Throne of Bhaal and going on the run across the planes. The player’s goal would be to gain followers (faith=power), usurp the divine portfolio of another god, and carve out a base of power in the wider multiverse. Your journey could take you to some of the planes we’ve never visited in a CRPG – Mount Celestia, Limbo, and Mechanus, for example – as well as revisiting old favorites like Sigil, though seeing the City of Doors through the eyes of a minor deity could be a very different experience. In a divine-level planar campaign, the player might stir up a civil war on Mount Celestia, conquer a layer of the Abyss, or assemble an adventuring party of divine avatars. Ultimately you’d take back the Throne of Bhaal, smack down Cyric and his friends, and establish a place for yourself among the pantheon of gods.
Mechanically, it seems like a critical resource would be your divine power, which would rise with your number of followers (humans, divines, demons, or whatever), the extent of your notoriety and influence, and the importance of your divine portfolio. (The concept of divine rank could conceivably replace character level.) You’d create avatars to go adventuring in the various planes, so death wouldn’t force a reload. And as you travel the planes, you might learn ways to change or improve the traits of your avatar, or to create multiple avatars, each with different shapes and abilities, useful for different situations.
That’s just some quick brainstorming, but I think a divine-level BG3 could be a lot of fun, and the Bhaalspawn’s story could certainly go on if developers wanted to pursue it.