Basically the pokemon you see in the poster Ash will/can get it. So if there is a Lucario, he will get Riolu and evolve it.
I'm the one sticking to Magmar, it's just me predicting it based off the ending theme with those little circles were Ash has caught one of the pokemon in them.
I dont think Ash will catch Galar pokemon for now, and if he does, it will be in the Galar region of course.
I dont know if the size matters, but he is hugging an Alola exegggutor in the poster, it could mean something.
Also, what makes a pokemon a special "mon"? Just popular or strong? I like pokemon that are not portrayed that much like Magmar. There is also a Magmortar in the sinnoh section in the poster, so if Ash gets a Magmar he is definetly going to evolve it. Also, Magmar can learn a wide variety of moves, and they can make him an interesting pokemon under Ash's command. It's just my opionion.
I'm still fine if he gets other pokemon, like Sobble and Grookey. For now, we dont have any hints for Galar captures.