What do you mean? The current systemn is great and should not be simplified. The UI could be made better, but the mechanic should stay the same/become more complex.
I'd prefer if instead of having to look at individual Pops, I could take various sets of data and combine them to narrow down my search. For example, suppose I want to see how many Afro-American Catholics are employed in New Orleans in the Labor sector. I'd click on New Orleans, click on Afro-Americans, click on Catholic, and then I'd see all the information specific to that category. I'd also like to be able to search on multiple groups, or in multiple areas; if I want to see the general state of the Scottish people in my realm, I could just click "Scottish" from the main menu. Or, perhaps I want information from several specific cities, or several specific demographics.
That wouldn't necessarily require getting rid of Pops, but from the player perspective, you wouldn't deal with Pops, you'd just be specifying whenever you need to gather information to make decisions. That alone would be enough for me to pay the price of an expansion.
As for my other suggestions, they focus on increasing complexity. Trade routes, for example, aren't even implemented; trade flows entirely based on diplomacy (spheres of influence) and cultural influence (prestige). I'd have a system more reminiscent of EU4, wherein you've got Trade Nodes linked together. Unlike EU4, though, your infrastructure buildings would just decrease the time it takes to move a good and exert a toll. This change in the final price would simulate the way that trade routes shape the flow of goods and allow for independent railroad and shipping companies. If you want to improve your trade, but diplomacy alone isn't getting results, you could focus on infrastructure. Depending on how well the AI can work with this, it could also be used in lieu of Supply Limits: you've got a limited ability to ship goods, so you need to secure key infrastructure (or protect your own) in order to keep your troops in supply. Occupations would be more about seeking specific targets that can cut supplies.
Diplomatically, the trade system needs more options. Let me set tariffs on the basis of each individual country and good. Like my idea with Pops, you'd just start with your general Tariff level, and then you'd click on countries and goods that you want to specifically target. Likewise, trade agreements would add a lot. In fact, I'd say that the Sphere of Influence system should be reworked so that instead of just giving you a captive market, you can spend your built-up Influence on "favors", like favorable Trade Agreements and diplomatic actions.
Lastly, companies (as in, organizations that own factories/infrastructure/resources and compete with other such organizations) and stock exchanges would make the economy develop more organically. It could even be tied into corruption mechanics (where you can be bought by a government, or buy a weaker nation's government) and labor disputes (companies having control over their own level of Reform, but also having the ability to violently quell unions).
