I posted a boss idea for a heavenly armor boss a while ago, got shot down pretty quick without people realizing the potential, like it could have been a Fiend Boss, the fiends just used Angelic Shields to form a mechanical angel they could control. They could add new enemies like Fallen Angels, which we really don't know anything about in SK, and have Angels be the ones pushing you to explore the area, it could be a DLC mission.

back to you though, I think your idea of a mecha dragon would be fun. The theme would be, mecha-beasts, but I'd say that'd be DLC mission material or maybe it could be the next prestige mission, who knows. Either way, your gonna have to give us more of the specifics. what else would you do, for the levels leading up to it, and the enemies you encounter, back story, etc.