I think that it would be neat if Warhammer II introduced some new infrastructure stuff.
Roads - Though most factions, in lore, don't really bother with roads, the option should still be there for people who want a tactical advantage. If I were playing Empire with 14142124124124 income, why the hell could I not invest in Roman Style highways?
Rural Improvements - Why would I want a farm in a city? I understand that the city is supposed to represent the province, but I think it would be cool if they were separate. Rural improvements could be dependent on city improvements to make things work. For instance, what if I improved rural areas to be farmlands, but that wasn't enough to make a profit, only growth. I could then build some kind of trade or commercial building in the city to facilitate exporting the product made and therefore earn revenue from those exports. It also means the raiding system could get fleshed out more. Raiding lush farmland? You should get some unit replenishment. Raiding a dwarf hold with lots of gold? All their gold now belongs to you.
Reformed Public Order - Why does giving my peasants beer make them happy? Amenities of course! But why is that the only option to make a province happy? There is more than one way to make enemy provinces unhappy! I think roaring economies, lots of protection, natural beauty, external threats [enemies on border/in territory], and some more I probably can't think of right now but make sense, should help improve public order. This creates a more realistic sense of how people would behave. Why would they need beer if they are richer than they could ever imagine?
Taxes - What the hell happened to my tax slider. I want that so that I can tax each province at different rates!
Growing Cities - I think the growth mechanic should greatly be enhanced. Right now, a province grows from a number of buildings and events, until every city is max size. That is pretty boring, and it means successfully developed provinces do not become massive hubs that you would expect them to be. Why is the core province of my Empire, Reikland w/ Altdorf, the same size as some wasteland in Nordland? I think there should be no cap on how large provinces get, [will talk balance in next points], and that as long as there is something keeping it appealing and growing, the cities should expand.
Squalor - Oh No! I don't know very many people who liked how much of a hindrance this was on their Rome II cities, I know I hated it. This would be more manageable, and now a hard and solid public order penalty. Squalor would run through some equation to reduce both grown and public order but would be its own, reduceable, number, that only grew as cities grew larger in population [not just for building a building]. There would be several buildings and techs that helped improve reduction in squalor - like bath houses, clinics, etc. Of course, the more 'civilized' empires would have an easier time managing this, and therefore be able to grow more. It means that the squalid and uncivilized Orks would not be able to keep up with growth, which is really understandable. This mechanic would be manageable, though it would eventually prevent a massive city from growing, balancing infinite city growth.
Immigration/Emigration - If people are not happy, why would they stay? Each different race should have immigrants coming and going, helping or hindering growth. Roads could massively increase the speed of this, for benefit or destruction. This would probably be public order driven, as people who are unhappy want to move somewhere that is happy.
Negative Growth - If there is too much squalor, or too many people are leaving, then population should go down. As population declines below thresholds, buildings should be abandoned, and city size shrunk. If population reaches too little [below the necessary to maintain 1 building], the city should become a ruin.
Politics - I think if politics were revisited, and civil wars were possible but balanced and such, it could be an interesting mechanic. Not really required, though.
Trade - There should be more to trade than having a product then selling it. Tariffs on imports for each product and excise taxes on exports for each product should be a slider. This should affect how much you make off of each being imported, but also affect the amount being imported. If your empire is super poor, you can't have huge tariffs or no one would be able to buy it and therefore nothing would be imported. On the other hand, if your empire is super rich, higher tariffs [though they may decrease public order], are possible, as people will be able to afford it. The same concept would apply to taxes on exports. Infrastructure, like roads and ports, should make trading easier, making exports and imports occur in greater quantity, making more profit.
These are just a few ideas, I am sure there are more, and I am sure I said a few stupid things.
I also doubt any of these will be acknowledged.