If Brook relinquishes his belt at 147 who would you like to see Spence fight?

I wouldn't be surprised if Brook drops his belt and leaves for 154, but it does create an interesting situation where Spence, his mandatory, has to fight someone else. I think the most logical step would be for Shawn Porter, the previous champion, and Errol Spence to fight each other.

Has the governing body eve had a stance on what would happen should Kell Brook give up his belt? That seems like the sort of thing that should have been talked about by now, but I haven't heard anything.