See, I'm thinking that when Gil and Agatha marry, that will merge whatever remains of the old Heterodyne lands with the Wulfenbach Empire (though I suspect it has already abosorbed them). So Heterodyne and Wulfenbach will be one kingdom, ruled by the two out of Mechanicsburg.
As for Tarvek, I suspect he will end up becoming Storm King, reclaiming a big chunk of his old land, and maybe marrying Violeta or the Green-Haired Princess. (The Storm/Heterodyne prophecy can be fulfilled by one of Gil/Agatha's children marrying one of Tarvek's).