Now, I've noticed that overall most IH-es seem to be currency-based. While that makes sense, paying out money for volunteer regiments simply does not. So, I propose a new system. A percentage value of each region's population is available to recruit. Once it is recruited, it cannot be replenished except by population growth, which is a different percentage of the population. The fewer troops you recruit, the more your population will grow and vice versa, as well as even versa vice. There are four types of troops.
Regulars
Volunteers
Conscripts
Elites
Regulars can be recruited from any region and are the standard, professional troops. They have average rate of fire and combat ability and have average morale, but they cost money.
Volunteers can be reruited from any region and represent the, well, volunteers who made up most US and Confederate armies in the ACW, as well as the regiments of many other armies who volunteered. They are free, are good melee fighters, and have good morale, but are poor shots, and the volunteering process makes forming these regiments take a while.
Conscripts can be recruited from any region and represent those pressed into service against their will. They are free, but suck at everything, and recruiting them will lower your national happiness. However, they also insta-spawn as soon as you order them.
Elites can be recruited only in your capital region and those surrounding it (adjacent). Any Regular or Volunteer regiment who does something awsome will automatically become an elite unit. They cost quite a bit to train and take a long time, but they have amazing melee ability, are great at shooting, and will never ever retreat unless you order them to.
Needless to say, this system is best for "modern" games, from the 1500s-WWI. However, it could also be used in ancient games, and medieval games, if modifications are made.
I also recommend a modified version of Fred's dice-roll-number system, but that would be in a different poll.