For spending, I'd dump the EP in favor of something more quantitative, like $1,000,000 as the base economic unit. Abbreviating costs for things to spend on as XM$, basically. Not *much* different from the EP, except that it's much more divisible and doesn't have a stupid name.
A manpower pool is essential, but rather than forcing players to deal with raw numbers, I suggest a qualitative slider, along the lines of Exhausted - Highly Depleted - Depleted - Acceptable - Strong - Very Strong - East Asia. It's obvious what kind of policies can effect this, the biggest being getting young men shot in the vital organs. You could implement raw numbers if you want, but it's definitely more work to keep track of, because are you *really* counting how much of the male population comes of age in each country in each quarter of a year? Of course not; you have a general idea and you extrapolate from there. Which argues for qualitative stats.
In terms of economy, I propose (Base/Industry/Trade) - (Debt Service/Upkeep), with income rounded to the nearest 1M$. This allows players to do concrete things, like "Lower Protectionist Barriers In This Fashion," which would (in a textbook scenario) lower Industry and raise Trade income, though how much in each direction would depend on the player's implementation. Base would be the income tax base, of course. Obviously keep a separate Treasury/Debt stat, but handle deficit spending (in terms of determining how much is acceptable) on an ad hoc basis, aka the PerfNES model.
Dachs already brought up the 'sophistication of industry' problem on #nes, which is...problematic. I think you can solve it though, by simply saying what each country can produce and where, modifiable with long-term reform and/or throwing a lot of money at the problem. Possibly an 'economy description' along the lines of the old army descriptions? In both AFSNES and DaNES II, the military descriptions changed incrementally over time, and I propose a similar approach for an economy description.
I also propose a qualitative stat which summarizes the current direction of the economy, providing the player with a general guide for whether his income (at least Base and Industrial) is going to increase or not. If you prefer to make it quantitative you could put the last quarter's percentage change in GDP, but again that's much more work for the mod in determining what percentile of GDP change translates into 10M$ increased/decreased income, since it would be situational for each economy. The one really tricky part with economy seems to be how mobilization would work, and how it would integrate with spending.
Keep factions. Integrate current factional goals INTO the stats. DaNES II was great with the factional goals, but presumably those change with time, and can do so quite rapidly depending on the political situation.
I propose higher-resolution maps for theater combat along with the generalized placement (by the mod) of corps to army-sized units and major fleets. It's just necessary, and saves the mod a lot of obligatory 'WHERE THE **** ARE MY TROOPS' questioning post-update.
Dachs might and probably will disagree with me, but I think accessibility for the proletarian player is key. And of course this all belongs in the NES development thread rather than here, but activity is activity, eh?