I'll explain the Pokémon of Litharreon.
In the years to come, the Pokémon Franchise will be bound to expand and "discover" new species. Regardless of this, Eventually in time, they will all be added to the growing collection of obtainable Pokémon of Litharreon. Rewards will be given for completion of each regional Pokédex's and there will be addtional sidequests to give more reasons to want new Pokémon. Not to mention the desire to "Catch-em'-all" now you will be able to!
Not all the Pokémon are obtainable currently, but they are being added with each addtional updated release. The Gen 5 Pokémon are being implemented with superb back-sprites, but it takes time. Eventually all Pokémon will be implemented.