1) Fixing the war on land (right now this is the most frustrating thing for me in this game). It's definitely worth changing, and in free patches, possibly in the form of open beta testing and receiving the most extensive feedback. And it's definitely a priority, because building houses is fun, but it gets boring quickly.
2) The correction of naval incursions is the second priority and also free of charge (they should be clear), maritime countries should be able to mess with the fleet in order to implement it. The navy can loot goods and convoys for the benefit of its country. Paid features - there may be some dreadnoughts, but not unstable ones, just unique ones, cruise liners, airships.
3) Paid DLC - unique laws for different top countries (it makes no sense to do paid things for conditional Sokoto), more variations of power units, the possibility of organizing co-financing of large enterprises and construction projects by states. Lending by states with variable interest rates. The possibility of buying up the property of debtor countries. The ability to impose pacts on countries for financing. Money should not go anywhere (after the conquest and annexation of countries).
4) I agree with the author above that investments should be understandable to the investor (the state). No one will invest without giving back, either financially or politically.
5) It is unclear exactly how the population is formed, we need a clear schedule.
6) A system of laws for everyone is possible in the form of DLC (by increasing the number of laws, the base adoption time should be slightly reduced): aimed at the navy, international policy, investment policy, construction policy (built by cheap migrants or professionals), laws on the distribution of essential goods in society, they are accessible to all by subsidizing to the right standard of living or each for himself, laws on scientific research (applied or fundamental), laws on urbanization.
7) Add products
8) Add the ability to accumulate goods in warehouses
9) Houses are the basis of the game, so there should be more different houses.
10) More research (and compensation for this is a reduction in research time), implement mutually exclusive research branches.
11) Travel of the population (transatlantic trips, trips to monuments and shrines, fairs)
12) Holidays in different countries (New year, Christmas, traditional holidays) and at this time consumption is growing and happiness is increasing.
13) Cabinet of Ministers/Royal Advisers.
14) To give the opportunity to independently create parties and select their ideology in the later stages of the game after 1900
15) Election promises in the republics
16) Military and other types of taxes
Do not abandon the game, at least until EU5 is released, and for a couple more years after its release until it is perfected.