I was originally hoping to tackle this as an intro programming project, but I don't see why someone else can't implement it first and I can still poke with it. (I like bullet points, so bear with me please.)
Okay, idea: Custom Difficulty setting.
Option during new game that allows control over:
- Alien Stats +/-
- Rookie Stats +/-
- Alien Battlescape aggression +/-
- Alien Geoscape mission frequency +/-
- Average Mission Map size +/- (like, between 4 and 8 or something)
- Starting Funds +/-
- Council curmudgeonedliness +/- (How harshly they grade you)
- Research time +/-
- Manufacture time +/-
- Facilities costs +/-
- Maintenance costs +/-
- Elerium decay
- Alien deployment sizes +/-
- Mixed alien deployments?
- Wound recovery times
- Craft repair times
etc.
Generally, in-depth control of things that you may not want a Ruleset for (because you are playing multiple games and don't want to switch it on and off)