Interesting question.
Vukovlad's idea is interesting as well.
My thoughts are more on sim structure than historical nations.
A. Economics.
I'd like to see some sort of simple resources involved. Industry needs ore and energy. If you don't have the primary, you need multiple of one or more secondaries. Don't have enough, then your industry doesn't run. Therefore you have to indicate who you are obtaining it from- presto a supply line exists.
This could make strategic alliances, and trade protection considerations more important in fleet development.
The economy could also have 2 aspects- industrial and manpower. Manpower allocated in large amounts to achieve something like building a dock.
B. Fewer real heavy weights and light weights.
We are blessed to have some folks that enjoy storylines and keep the small countries running.
But if you figure 1 factories DD/Es, 1 misc, 1 infrastructure, 1 for aux, 3 for cruisers and 3 for capital ships. Then in 20 years you can have 60 DDs, 12 CLs, 4 CAs, 8 BBs/CVs on 10 factories..hey that was the original baseline.
So I think all PC nations should be limited to say 8-16 factories, with larger/smaller staying that way, unless somebody really wants a smaller. The bigger ones should have part of their factory base dedicated overseas, or dependent on imports.
C. Ports, not docks/slips
Right now I think much of the investment for a drydock can only be rationalized as going to the cranes and machine shops, plate making , gun foundaries, all the infrastructure dedicated to that. However none of these run flat out. I think ports should be the big investment sink, and the docks or slips there relatively cheap.
D. Generic tech limits
Set broad tech limits, make it random if in any given year the game progresses to the next one.
E. Not necessarily maintenance calcs, but some sort of limit on overall size based on population.
F. Forced war...or threat of same. Or some heavy hand on diplomacy.
For some reason it bugs me to see big blocks of adjoining nations all harmoniously joined against the outsider. History usually sees adjoining nations fighting, or considering it.
Maybe a default diplomatic status, and you can only adjust it up once a year ? Early on that would preclude trade so you make friends with different people... Not sure how this would work.