I'm not sure how you get a permanent Hunnic state. It seems to have existed solely as a parasite of the Roman Empire, with few serious resources of its own to allow it to stand independently. Once the Eastern Romans stood up to the Huns they quickly moved off West to demand resources from that area, but even there they suffered defeats. Even assuming more successful Huns in the West, ultimately they're not going to be able to continue existing in the way they did- their supremacy over various other peoples in their confederacy was assured solely because of Roman gold.
You can get a longer lasting Hunnic state if it becomes smaller and more concentrated, I would say. One good idea is to somehow have Constantinople decide to keep paying the Huns subsidies, perhaps to keep out another power. Having Attila die a little later, and his empire pass on to just a single son can help with this, though I doubt even then the Huns will be much more powerful than the post-626 Avars were- that is, the dominant power in Pannonia, but not one that can threaten all of Europe at will.