Out of all those Gengar and riolu seems the most likely as both of them have been constantly appearing in merchandise. The only thing we've seen that even hints at litwick is a chandelure appearing in merchandise once. The other starters are obviously gonna appear and most likely be on the main cast but there's no telling a far away that isAsh's two mysterious Pokémon are either Litwick/Gengar, Grookey, Sobble (or both.) and Riolu.