My comments on each speculation:
1. Evolution of Lucario. The pelvis/legs/hips are a lot like Lucario, and so is the arm we can see, albeit missing a spike. Also, that is likely a snout so this is probably canine. If it's an evolution of Lucario, it's going to end up a hairy/fiery grizzled version of Lucario. IMO, Lucario does not look like a fully evolved Pokemon. Just look at Magmortar and Electivire, there's definitely room for Lucario to evolve and adopt a floofy design. Also, while you can't see an outline of poofy shorts, there are two grooves in its hips where the shorts could end, but they would be form fitting.
Also, while many are asking, "why would they release an evolution as a first 5th generation Pokemon, why not a stand alone?" historically Pokemon has released preevolutions first. For the 3rd gen we got Azurril, and in the 4th we got Munchlax. Lucario was the second 4th gen Pokemon to be released, not the first.
Plus, Lucario was a main character in a movie before the 4th generation was finished coming out, and so is this one, so there's a paralell. But this could just be Pokemon using a formula.
2. Night Evolution of Riolu. I like the "Werecario" theory, but it seems doubly complicated that Riolu would have to hold an item in the night, or just evolve in one location during the night. I almost think it shouldn't matter if it couldn't evolve before, it wouldn't be the first time Pokemon ignored continuity.
3. Same social standing as Lucario. This sounds likely as well. Maybe a fighting/ghost as opposed to fighting/steel? It should have more red in its design then, to paralell Lucario's blue.
4. Evolution of Mightyena. I highly doubt that. Mightyena is one of the basic Pokemon in Ruby and Sapphire. It has the same standing as Venomoth - people just like it more. Why would it evolve to be the star of the thirteenth Pokemon movie? The logic on this one is just way off.