An awful lot.
For example with the variable manipulation - I wont know this for sure till 2.9 drops and I try it - one thing i'd want to mod in is a ship-size cap for the smaller ships (BBs, CVs, DDs) to force more mixed fleets.
By being able to read in your nation's current naval capacity (and used naval cap percentage - and its reciprocal) you can set limits (e.g. no more than 20% of a fleet may be battleships) that the game can use to block ship construction.
- So if your fleet is already at 20% BBs, building one more is either A) blocked as an action or B) puts you in breach of a GC resolution (if one modded one exists setting ship-size limits). Effectively letting you create naval treaties and encouraging the use of "non-meta" ship sizes.
- You could also give militarists special bonuses. By default, for example, each ethos could let you build up to 20% BBs in your fleets, but militarist governments raise that to 35% and Fanatic militarist govts [and gestalts] increase it again to 60% cap - or mil-FPs [and DEs/DSes] raise it further still to 100%.
- Suddenly fleet compositions now differ with nations ethics, whilst militarists can field capital-heavy fleets, most other nations may be stuck with smaller classes. Pacifists could even have this go the other way, reducing their larger ship sizes down to 10% - so cruisers or corvettes will end up being their main fighting vessel.
And thats a pretty basic use of just converting naval cap in to a variable, computing fractions and setting limits via flags, country modifiers or "game rules", which could have a large impact on the game's warfare.
You could do many more exotic things, too, with a little creativity and maths ...
