The problem with Gunblade is that it's an AP item that happens to have AD

To put it succinctly.

Instead of being a giant stat-stick, why not make it an item that would actually be attractive to true hybrid champions?

Scale back the raw stats on it a bit (the AP in particular) and make the active scale off both AP and bonus AD. Add an OnHit effect that deals bonus physical damage based on your AP and bonus magic damage based on your AD. It would still be attractive to true hybrids like Jax and Kayle while becoming much less attractive on Mages just looking for another spell to add to their rotation.