[HoneyBadger]

I'd just like to add that I'd really like to see a progressively stronger PD rather than just a numerical increase.
What I mean is, at PD 1-19 you get a few weak-type troops and a basic commander, and then when you hit 20 you get another commander and some better troop-types, and then...that's it-for the rest of the game, PD is just a small hole you toss money into, to make the hole a little deeper, hoping your enemy will fall in and not be able to climb out again before you shovel dirt over him.

That's how things stand now.

I think it would be a lot better-especially for nations that start out with weak PD-if they continued to get additional commanders and better troop types as PD increased.
One way to do this-and keep it balanced-is to push the second commander/better troops back to 30 PD, and then add another commander/better troop type at 60pd, 90pd, 120pd, and 150pd (increasing max PD in the process, in case you didn't notice, for those who might want to focus on it, and would now have a reason to.)

This means that, at the maximum 150PD, you'd be fielding 5 commanders and atleast 5 different types of troops, making the act of building strong PD a solid and interesting strategic choice (whereas now it's just a place to dump extra gold until someone comes along and wipes your province out anyway).

It would make castles and core-provinces more important, longer games more interesting, and also blunt the edge of sneaky-quick nations like Helheim. You could even design interesting infrastructure-nations specifically around their PD.