What I liked about the first two Grandia games was that they were personal journeys for the protagonists. For Justin, it was a coming-of-age journey and convenient plot devices be damned, we saw him come of age. For Ryudo, it was a journey of redemption; sappy endgame sequence be damned, Ryudo did find that being a festering cynic is not quite synonymous with being a realist and that there are positive aspects to reality too. I don't recall any journey theme for whatshisface in Grandia Xtreme. And Grandia 3's Yuki... it could, nay SHOULD, have been a simple journey of a boy chasing his dream and leaving the nest. A good theme to tie into that could, nay should, have been his mom's psychological journey coming to grips with her son becoming a man and not her little boy any more (though any mom will always view her son as "her little boy" no matter how old he gets.) A simple tale of a boy, his plane, and the changing dynamic of his relationship with his mom. But it definitely didn't happen that way.
My wishful thinking would be that the next Grandia game returns to form where the theme is a personal journey for the protagonist. And even more wishful thinking would be to have the next Grandia game have a female protagonist, but that's probably stretching it.