Hmm...many speculations. I've only gone through six pages and got tired of reading the same responses, so I want to post my own.
As far as I've read, there seem to be a few main speculations for this new 5th Gen Pokemon: Lucario, Mightyena, Eevee. I am going to highlight these Pokemon in order of the least reasonable to most reasonable in my opinion: Eevee, Mightyena, Lucario.
EEVEE
An Eeveelution is highly plausible seeing as new ones have been introduced in nearly every generation save for Gen. 3. This would mean that, since an Eeveelution hasn't been introduced in a Generation, one is overdue, hence a Gen. 5 Eeveelution. However, Eevee already has status in the games due to its very complex nature of evolving so its importance is pretty much set; creating an Eeveelution to star in a movie would simply increase the already-high status of a Pokemon which is actually not important. Yes, Dragon is the only Special Eeveelution left, but we may see that in the games, not the movie.
MIGHTYENA
This seems as plausible as a Lucario/Riolu evo, though on a lesser degree. Mightyena is based on a hyena, but the Pokemon looks like a werewolf. So, it would make sense for it to be Lucario, correct? Unfortuantely, Lucario is in the same situation: it's a jackal, not a wolf. Anyway, lycanthropy affects all species, giving them characteristics of a wolf in the process. Lycanthropy in a hyena would give it rather intense canine appearances, which is what the Pokemon looks like. Also, Mightyena doesn't have much status, therefore giving it a lead role would be a sudden turn on the Pokemon community and be rather interesting and intriguing. Another thing to note is that in the movies, Pokemon from the same number-class generation (even, odd) have been featured in the same movie. In the DPPt Movies, we saw many Gen. 2 Pokemon, an even number like 4; in Manaphy and the Prince of the Sea, we saw many Gen. 1 Pokemon, an odd number like 3. Therefore, in Gen. 5, we would see many Gen. 1 or 3 Pokemon, such as Mightyena.
LUCARIO
This seems the most plausible. Since Riolu evolves into Lucario during the day, what happens at night? Lycanthropy happens and, this time, it would affect a jackal. The form seems most plausible because there are many things highlighting to that fact: the paws, the tail, the slim torso. Another thing to note is the excess fur, which could hide the metal spikes portruding from the chest and hands. Also, Lucario also has status in Gen. 4 as an OU Pokemon, so an alternate evo could raise it to Uber status. Now you might be wondering "Didn't you say raising high status was bad?". Yes, it is, but only if you've had such status for 4 generations, like Eevee. Lucario has played a major role before, so it would seem like a likely candidate.
These are simply my observations. I purposefully did not include the notion of it being a completely new Pokemon because that's pretty damn obvious. If it were a new Pokemon, it would be related to Ash in a sense, somehow. Maybe. However, in my opinion, this new Pokemon is most likely to be a Riolu evo at night. Lucario is already connected to Ash because of their use of aura and Ash's resemblance to Sir Aaron. A dark Lucario? For me, highly plausible. :/
