1. State the current situation: As of right now only total population with modifiers is shown
2. State the problem with the current situation: Since modifiers only refer to the base population of your nation and not the total population there is no easy way to calculate on the fly how events, trades, improvements will affect your total population count thus making it harder to come to an informed decision. There are calculators out there that can do it for you but not every player knows about these. You could also try to calculate your base pop by subtracting your modifiers but c'mon that's a bit tedious and could get a bit out of hand. A practical example is that right now most middle sized nations have to buy Border Walls until they see a drop in income and then go back and destroy the last one they bought to go back to the optimum number. That's just wasting money when they could figure out with a little math exactly where to draw the line.
3. State how you suggest to changing it: As with land and how it shows purchased/modifiers/growth you would do the same with population. ie base pop/pop from modifers/pop from land
4. State why your suggestion is better than the current situation: I believe this will give players a more informed view of their nations properties so they can make better choices when it comes to the effects that certain wonders/improvements/resources/events will effect their population and thus income and then can make better decisions about how to build or alter their nation.
As an aside you can add it to the extended display just so that it's there. I would also only make it viewable by the player.
