I took the survey and have a few suggestions for SP (nearly 3k hours):
1. Japan, the USA and SEA need a face-lift to make the Pacific war a thing, because right now the Pacific War really isn't a thing. Guadalcanal never happens. Both Foci trees are ancient.
2. Food is something you should consider including along with division limits for performance. No one is fielding these many divisions and feeding them.
3. If the AI can't handle island hopping make pre deploying and staying I'm islands more expensive for the player in terms of supply.
4. Since equipment tags became a thing the US starting fleet of cruisers and aircraft carriers have the wrong tags, the Wichita isn't a light cruiser but has the shield tag so it doesn't reinforce fleets with heavy cruisers (the turret tag). Same thing with the Lexington which should have a CV tag but doesn't(has a turret tag instead of plane). Doing this every game gets old.
5.The attack on Mers al Kabir never happens if you play as the US, so the Churchill sequence of decisions ends with emergency arms deliveries. This is due to the UK having so many decisions but not enough PP to actually take them. Same thing with invading North Africa.
6. The USSR never follows through on Potsdam, because there is no Potsdam and Yalta doesn't do what it should.
7. Research bloat has hit the USA, you never field everything you could because you lack slots to do so.
8. You don't actually do anything with spies in the US till you're the spymaster so you don't do operation Bolivar.
9. Having Iran join the Axis because the USSR and the UK can't topple them in SP is stupid.
10. Having the DR, El Salvador and Venezuelq join the the Sphere or the Axis when the allies are winning is hilarious.
Edit:
11. Having the USSR lose Stalingrad because it's garrisoning the Kharabosk coast is silly.
Edit 2:
12. Lend lease is broken especially to China.