more dinamic badboy system. Something like EU III,where tehre are some factor wich changes the max badboyd (for example, max badboy with genova convention +2 (or -2,whatever),genova convention -0.1 monthly)
Also, if you are in a badboy war, you should lost badboy in a normal way. Its very exploitable if someone has for example 26.00 of badboy and declaring war.. because they need a lot of months and not only 10 or so to go below 25.. so when they win or lost that badboy war they have another one,or two,or three in a row.
If a country is doing well, their pops should not migrate (not only attract inmigration). Also, if an american country is doing porrly they should not attract any at all, but thats is my opinion.
If a country has slavery,they should capture some pops in every war when siege, but with a badoboy cost in every war (+0.1 instead of -0.3?) (also, a lower badboy cap if they have slavery))
Head of state and head of goverment should be nice in every election,with bonus and malus. If there is too much corruption in the country they should be bad rulers, and viceversa.
Would be nice select multiple provinces in a row and make them to build something. (rusia..). Also canceling a building ordering by you should be possible. (missclick)
also tech bonus if you are researching one tech that your neighbour alread reasearch that and the next tier
if you have a positive war score and you conquer a lot of the other country, but your country only have like 10% war score (usa conquering british canada), they shoud give up some provinces there if war is long enough.
and, THIS:
also please add "set rally province" option for troop and ship construction. it is so boring to collect and merge all those ships and troops