I reserched again, and its Giradina, there is no other, cuz Hiidoran, or w/e that fire steel toad pokemon's name was, cant be the star of the game, cuz his role is the same as Latios and Latias, and the 2 legendaries who act as Latios and Latias are the Fire/Steel toad pokemon, and the crecent pokemon that looks like a porygon, and also,
they both have genders, No pokemon on the cover of new gens had a gender, and the only pokemon who had a gender was Charizard etc. for the "bring back the old red version" naming it Fire Red Version.
So the point is... Giradina has to be on the cover
if they make the third part to the series.
[[My info was summerized by wiki-pedia into my own words]]