Real multi core support, so more calculations can take place with out slowing the game down. These area needs improvement :
AI : If AI had its own core, allot more could in theory be done. No need to simplify the scripts for the ai to run faster.
Combat : New values can be added to combat for more realistic combat, with out slowing the game down (Torpedo attack, meele, medium rang art, long range art, etc)
Detailed supply travel, no need for simplifications like the devs admit in the faq for supply.
The best would be if we need not wait for hoi4, a multi core exp for hoi3 with some other new stuff.