I don't understand. You didn't like the Trundle rework, so you're suggesting they do the same for Sion? It's not as if Trundle was super in-depth either before his rework; as I recall most release champs only have a few lines. So why Freljord?
Every champion has SOME sort of identity even if it's mediocre at best. For Sion, he's got the undead warrior of Noxus thing going for him. So we make him from Freljord? What purpose does that serve? We make him a frozen corpse instead of a stitched up zombie with "cannibalism"? Additionally, Riot wants to majorly rework his kit so most of his abilities will probably change completely (say what you want about the Trundle rework, but the tweak they gave him both retained his gameplay feel AND made him better overall). You might as well make a brand new champion at that point.
I just don't get why you think we should change where Sion is from. Noxus is an iconic location that centers around perseverance and prowess in battle being more important than birthright--and Sion perseveres even after death. Noxus has a tradition of reviving their most beloved heroes, and Sion is one such example; but he's a shell of his former self, only having a single purpose: to fight for Noxus. How can you look at that and say: "Nope I don't see any opportunities for characterization. Better make him from Freljord because Lissandra needs a flunky."