The only flaw with picking governors like great people is that now you either need a unique promo tree for each of them, or else they need something unique about them, and that can be tough. I do think the notion of being able to recruit multiple stewarts would make sense - as long as they're balanced between each other.
One thing I wouldn't mind seeing would be to change each governor to give them a direct benefit for each promotion level, on top of their other bonuses. So, for example, Pingala would be "+5% science/culture per promotion level", and then the rest of his promotions would stay the same, for example. Maybe Liang instead of the builder charge would be "+10% production to builders (or maybe all non-combat units) per promotion level", with the +1 charge per builder tacked on as one of the promotions. Magnus could simply be +10% extra chop bonus per promotion.
That way, each governor would start very weak, but would grow stronger the more promoted they become. And I would definitely like some later promotions on the governors to truly alter bits of the game. For example, if a later Victor promotion allowed him to instantly set up in a new city (once per turn), then suddenly some abilities become more interesting. I mean, to instantly teleport him to a new city to give them +5 strength, an extra city strike, and not being able to be put under siege may actually be a useful skill, instead of now by the time I move him to a city and he takes 3 turns to set up, I've probably either cleared out the threat or lost the city. Combine that with other improvements (instead of only getting +5 on defense, give it on offense too), and then suddenly you now basically have a new great general that you can move along your front line.
Although in defense, I actually just recently used Victor and I think he saved a city for me. Gilgabro declared a surprise war on me and started taking down my city, but just got a governor title and rushed Victor to the city. They got it down to red, but between his +5 extra strength and upgrading to my first swordsman, I just barely held on to get my troops back to push them away. I think without that +5, there's a decent chance he may have captured the city. But this was the very rare time that Victor helped.