I think Ghirahim might become the "anti-master" sword from this artwork: View attachment 19269
Notice how Ghirahim has a red crystal on his belt, the anti-master sword has one on its hilt. The goddess sword (the one that fi turns into) has a blue crystal on its hilt, and Fi has a blue crystal on her chest. It makes some sense, but we'll see.
Yes, I think so to. Fi becomes the Master Sword, Ghirahim the Anti-Master Sword. My question is why we have never seen the sword in games later on the timeline?