Portrait Regulus Black
I find it very unbelievable that Regulus "Momma's Boy" Black wouldn't also make a portrait before he walked to his death, even if it was just to help Kreacher. He was willing to die to get rid of that horcrux, he could've made a portrait.
So, because Regulus Black is amazing, he makes a portrait and keeps it in the infamous Black Library. No one goes there for 16 years, until Harry gets bored in Grimmauld Place (some time in OotP) and wanders in the room. And suddenly he hears a voice similar to Sirius's, but with a touch of regality, say "James?" or "Potter?".
And he finds out about horcruxes and SWM and the Incident way before he did in canon. Harry becomes friends with Kreacher and learns dark magic from Regulus.
But then Sirius gets confused, so he decides to investigate. And there he finds his brother's portrait LAUGHING with his godson. And you can decide what happens after if you use this idea.
-Alexis