PD does work fine the way it is, I totally agree with that-it's a great idea, one that inspires me to argue that it could be even greater-If it sucked, we'd have a lot more threads about it. I'm just saying that it could be somewhat cooler
If nothing else, I'm just hoping that the Devs will allow it to be modded more dynamically, sometime in the future.
I don't know that there needs to be an arbitrary limit on it, either, except that I'm guessing at some point it would start to drain processor speed. Even if your 5000 PD just consisted of 50 Jotun Jarls armed with artifacts, the computer still has to keep track of all those stages and transformations. 2 stages, in my opinion, are a little low, but more than 6 or 7 would be a bit much, even for me. I'm only saying that it makes sense to allow fortresses to give you a bigger upper limit on it. It doesn't add to micromanagement or take away from balance that way, since you'd still have to pay for it (and even reaching PD 125 is quite expensive, let alone PD 150.).
And having unlimited PD scales not only means more processing, but more micromanagement, and a more crowded screen. PD shouldn't be precise, you should still need to recruit units and support it-and understand what you have, and conform your tactics to it-to really make it shine. It should never do the work for you, if you want to get the most out of it.
And national PD units should never be elite, they should just get better, in a limited way, over time. Having elite units added to your PD-the Draconian example for instance-would be a rare and special bonus. It should happen occasionally (in my opinion), but it shouldn't be something your enemy will always expect.
For one thing, this is another way that those units we never recruit could show up in the game. You wouldn't spend money on them, but integrate them into your PD, and suddenly they're nice to have when you need them.
As far as a self-growing/shrinking PD, that would be nice, but it would also be difficult for the player to manage, and a headache to program-and for that matter, again probably a big drain on processor speed.
I'm not in favor of turning PD into some all-consuming beast, just that it have some of the variation and surprises that other aspects of the game currently have, and that it be complexly modifiable.