transport cost: say you have oil in saudi arabia and factories in turkey and you would have to ship the goods to Turkey which would take time and money. So a realistic transport system + locations. So if every unit has a location u would have to transport it. That would add a fun logistics part to the game.
running costs: Every building costs u money to run (bills, staff, rent etc) and you can also shut it down when you dont use it.
staff: different quality staff for different areas and different wages. So lets say u decide to give bonuses etc to increase the production of your buildings. Higher quality workers work better and cost more etc.
So just a few things i was thinking about, i m sure there are ways to make these ideas even better since I didnt really think about it for too long

btw how open are u to new ideas and improving the game? or do u just like it the way it is. I would like it as complex/realistic as possible.