Ooooooooh my goodness, Jade Empire!!!! Can't believe I nearly forgot about it. It was BioWare at their most creative imo with stellar writing and worldbuilding, and a lot more fun combat than KOTOR. BioWare made Mass Effect after this game, so not surprising that Jade Empire never got a sequel, which is a damn shame.
I'll also throw in Phantom Dust as another original Xbox game that never got a sequel.
You called down the thunder, now reap the whirlwind