You can't just change the Bow into a Darkin and be done with it.

You can't do it without creating another Kayn, or another Pantheon. Let me explain.

As seen in the Varus comic, where The-Darkin-Properly-Known-As-Varus goes on the Noxian killing spree, with Kai feeling really angry about it, allowing it, not fighting it, the Possession of a Darkin is pretty tied with their Host's feelings/spirit. Kai was angry, and Varus used those feelings to try and gain full control. If it wasn't for Val, they'd both be consumed and lost.

Now take a look at Kayn, another human host is constantly fighting against Rhaast, for control of the power within. He's strong willed, determined, and pretty powerful all things considered, even without Rhaast. He is at a constant 1v1 against Rhaast, a constant battle of will, to wrestle control away from each other. And that's the thing, Kayn is actively fighting against Rhaast.

Now take a look at Varus after the death of his family. He's grief stricken, heartbroken, and filled with a desire for vengeance against Noxians. Going by how The-Darkin-Properly-Known-As-Varus tries to manipulate Kai into giving into his anger, how do you think Varus would fare? He doesn't care what happens to him, he just wants revenge, so all the Darkin needs to do is whisper some sweet lies, and then take control.

Suddenly, we have another Pantheon, where the Darkin has fully taken over Varus, making it a completely new character with a now irrelevant backstory. The only other possibility is that Varus tries to fight for control over the bow, but then we just have Kayn-but-with-a-bow. It's even more unlikely for the second to happen due to how desperate and broken he is for revenge in the first place, it'd be incredibly easy for him to take over.

Darkin are infinitely more complicated than whatever was in the Pit of Pallas, you can't simply copy-pasta a Darkin over it.