Apologies if any have already been mentioned as someone who has just returned to AOD...
1. Intelligence
A revamp of the intelligence system to simply assign a set amount of money to spying operations against each country, rather than accumulating spies, which then is reflected in intelligence obtained on that country. (More representative of a combination of technical intelligence, diplomats, undercover agents, secret reconnitoiring, etc)
You should automatically have rough estimates of military units for countries that have open societies/democracies (unless at war). For example - number of capital ships was known in the pre-war period.
2. Diplomacy
Money should be used to pay for military and civil expenses, tech research, etc rather than the rather hazy concept of "buying" influence and other diplomatic initiatives.
Influencing another nation should come out of some national influence pool, constantly recharging over time, dependent on a nation's IC, military size, belligerence, etc to better represent diplomatic influence.
3. Standing Army vs Drafted Army
Better differentation would be nice and more realistic, it seems there is little incentive to bother with drafted army.
A fully drafted army should be extremely cheap but ineffective, whilst a standing army should be horrendously expensive but very formidable.