I wanna throw my ideas and combine it with some ideas I liked there.
1 Mercenaries as proffesional soldiers.
Many of you thought, that "standing armies" of EU3 are something like levies, peasants from fields. What if, we changed that to reflect this problem?
Mercenaries: mutch better than regular soldiers, but more expensive. But:
a) you can change your regular soldier to mercenary soldier (for a price of course), in addition to hiring mercenary soldiers in standard way. Making proffesional soldiers this way, those would be replaced very slowly (those regaining men from manpower, forgot that word..). If one can change those back to levies, thats optional.
2. I realy liked the idea of realy low soldier limit, that would be slowly builded through barracks etc. This would make buildings mutch more important strategic decision, as well as colonizing, expanding etc. But no starting limit is too harsh. Something like: 1 for every province + 5 flat bonus.
3. This idea wasn't mentioned there. Mercenaries have their niche even in this game without modification, well, without serious modifications. Colonies and new word. Hiring native mercenaries here should be possible and would make mercenaries realy important and usefull.