Instead of how many new troops you can recruit each year, it might be interesting if there was a cap placed on army sizes based on number and population of provinces owned (and maybe the cap rises after DOWs, reflecting the fact that standing armies were small). This could be breached, of course, by hiring mercenaries if you had the money. But it would be nice if there were fewer times when Navarra has 80,000 troops during peacetime.
Privateers would be cool if they acted like Pirates. You could post them off other people's colonies and watch them bleed those economies dry! Muahahaha!
And just to add to the first idea, provinces which have nationalist revolt percentages (in other words, conquered within the last 30 years) would not add to your total manpower cap until the 30 years had passed.