Having pop-type troops seems pretty straightforward and interesting.
Getting a holy caster when there is a temple built in the province just makes sense. Getting a caster type and/or magical creature when there is a lab in the province also makes sense.
It would mean that the few small changes that can be made to a province actually matter for something.
Personally, I'd like to go a little further and be able to designate some basic commands for the province defense. I've played a few nations where the defense default configuration is so bad it nearly kills itself. Being able to set where each troop type will start (even if you cant form groups on command, just the defaults) would be a huge improvement in how useful the PD is without increasing the amount of power it should have based on the troops it is using.
I have literally seen a province defense do enough damage to itself to route itself before the enemies could do hardly any damage at all. That was incredibly pointless.
Several tiers of troops seems pretty easy, it should only be a slight modification to what is already in place.
A single screen to adjust starting positions of each unit type and their basic orders should be easy as well. It could be as simple as the normal battle interface and a button to click on like research. Move the bits around and that is how all of the PD work. Being able to give a 'hold and attack' order to fliers would be nice, 'fire enemy cavalry' to crossbowmen would be handy, and being able to guide the casters a little is practically a must. On the last point it gets kindof frustrating again to know that you shouldnt research above a certain level in a certain path because it will cause the PD caster to use a fairly worthless spell, sometimes all the way to unconsciousness.
Using pop-types would be a larger, and more complicated change. It probably isnt worth the programming time it would take to implement, but it is definitely interesting
I do like the thought of putting some extra options in with pretender creation. But then I also like the idea of doing something similar with castle building and troop features (such as: 100 pts - the rage within your pretender is felt by even the lowliest of his/her disciples, all of your sacred troops have berserk 2 and the rest have berserk 1 (this stacks with any berserk they already possess) ). But then that kindof stuff could also go under a whole new research category and be something of a 'national buff' with each nation being able to only have a certain number of them active but possibly being secret. Lots of potential there
I definitely like the idea though. Province defense can be kept pretty simple even with changes like this. Mostly it just increases a fairly important part of the game to much more interesting levels. For those who dont wish to bother with it they can still just put some points into it and ignore it without extra consequence.