Possibly a new military unit (like tanks or aircraft), not sure how much it would cost however it would certainly be quite pricey. Each aircraft carrier would allow (idk, say 20?) aircraft to fight during naval battles. Just the basic idea, if you would like to add onto this, post below
Just a quick suggestion that shouldnt take much time or effort to add but would be interesting:
Hopefully, many of you have seen the average trade price history graph on P&W. Just thought it’d be a cool idea to also have a graph of the past 24 hours. Might add to the game a bit.