And the japanese official website of Golden Sun: Dark Dawn confirms, that Isaac will be definitely a side character during the journey of Matthew and Co.!
Because his silhoutte is shown in the characters section.
And a silhoutte of an old man is shown too, maybe Kraden?!

(But if I can recall really... Kraden must be dead in Dark Dawn, right?!

)
You can as well hear three samples of the awesome OST from Motoi Sakuraba in the japanese website!!
CLICK here!