They already have Isaac as an assist trophy, so I see no reason why they shouldn't go further in integrating the series into it.
And I just feel that since Isaac has been an integral part of both games, he should have priority over Felix, even though Felix is still really awesome.
Isaac could use catch or something similar for his grabs, pound for his grab attack, then use various sword attacks for his Standard Attacks, and for his specials use... An unleash for his weapon, a Djinn strike, or some other Psynergy... I haven't thoroughly looked at every aspect of the game yet, but something similar would be nice.