I mean, the chance to exist as an actual Avatar of a God - the God of Murder - and walk the earth fueled by that as a power.
You get to explore what it means to be an Avatar - filled with the divine essence of the God of Murder - and yet you have your own individual will.
Once we defeat the Absolute and free the God of Murder - I hope we can continue to play as his vessel.
I bet you gain the ability to transform into The Slayer.
And, if they ignore Abdel Adrian, given each Bhaalspawn was a piece of Bhaal that re-amalgamated into Bhaal, you can imagine that the protagonist in BG3 is a semi-continuation of Gorion's Ward.
