I could probably do OK as a modder, have enough skill, but am too lazy or won't take the time. Some cool stuff, though, would be:
* A more 'realistic' or 'historical' Draft/Mobl'z routine/path.
* A mod where Lee accepts command of the entire US Army in April of 61.
* A mod where Napoleon III goes barking mad and declares war on the US in the spring of 62.
* The Union player can trigger FI as desired.
* Scheduling Monitor and any ironclad builds for Mar62, not before - both sides.
* I'm sorry, I've beaten this one to death, but an Industrialization component that actually is a worthwhile option that can make a difference, especially for the South, wherein, if they are very, very good at the front, then a modest commitment to Industrializing pays huge dividends.
* Having Signal units do a bit more, like perhaps increasing MTSG probabilities. Have Engineers perhaps speed up the column's march speed a tad, or nullify or reduce the speed penalty for Arty units.
* Have Pinkerton or Belle Boyd be a tad more useful.
* Reduce FoW a bit for the Union in certain circumstances, to reflect the intelligence reaped by slaves who informed.
* Deliberately making Cohesion hits or Supply problems for Southern irregulars horribly debilitating when north of the Ohio or Potomac, except perhaps southern IL and IN, and parts of MD. The Loyalty matrix of rules isn't a bad approach, but under the present structure, it's much too easy for Jesse James to tear up tracks in Springfield, IL and still carry on with little to fear from the oppo - the guerillas will be stopped sooner by Supply than Union cavalry.
Just some ideas from a fellow with a keyboard.