I could see this being very strongly thematic-if for instance you were to Prophetize a Knight of Avalon, and he transformed into the unique unit 'Arturius Rex'-an immortal commander with a magic sword that turned it's victims into stone-Prophets could be really deeply tied into a nation's overall character. You could have dozens of neat little easter eggs.
I'd love to have the ability to mod this in.
I would absolutely add this to one of my Cimmeria units, so that if Prophetized he turned into Conan the Barbarian
It's unthematic, to make him a priest, but I could always give him Holy -3. No Holy magic would make for an interesting Prophet-you'd still get the sacredness and the Dom boosting, ofcourse.
Niefelheim Jarl ought to turn into one of the named Jotuns-like Utgart Loki, Bergelmir, Surt, etc. Surt would be fun--suddenly, instead of an average Niefelheim Ice Jotun, you've got a Muspelheim Fire Jotun to try to fit into your strategy.
Niefel Jotuns are just too much fun not to see more variations of them.