Basically, I see this taking place in between Season 1 and Season 2. A very humanoid-alien invades the world that TDl takes place on; Euclid. And he has a goal; imprison the entirety of the humanoid races on Euclid. Humans, nekos, demons, and even angels.
This character would be named Judge Tyrannos. He would be an antagonist on the ranks of say Xeren, but less powerful. His main asset is his strategic mind and his equipment rather then raw strength. He has a planet-shaped spaceship/prison named the Iso-Cube. And, a pistol and a motorcycle, but I'll expand on it more if I get the ok to make his character page.