Well, as a matter of realism, you’re obviously correct. The only way conquering and re-conquering Norway could increase its economic value would be if players were looting it each time, and if you tried looting 4x the country’s GDP in 1 year, then the country’s IPC value would probably drop to 1 or even 0 IPCs/turn for the rest of the game. Not to mention you’d be likely to get more resistance from partisans and guerillas who suddenly have nothing to lose, because their families are rapidly starving.
As a matter of game play, I have concerns. First of all, if you have to wait a turn to collect income from a territory, then the value of ‘light trading’ goes way down. Under OOB rules, if I attack your 1 infantry in a 2-IPC territory with 2 infantry and 1 fighter, knowing that on your turn you will return the favor, then statistically speaking, I can expect to lose 2 of my infantry, kill 1.5 of your infantry, and collect 2 IPCs, for a net gain to me of 0.5 IPC. This encourages light trading, which tends to bleed out players’ infantry reserves and reward players for taking the strategic offensive.
Under your rules, neither player would get the 2 IPC bonus during light trading, which means that if your opponent has the reserves to counter-attack, then light trading will cause you a net loss of 1.5 IPCs. This discourages light trading, which tends to help players accumulate enormous stacks of infantry and rewards players for turtling and waiting for an opponent to attack first.
Another potential problem is that players could get discouraged and stop seeing the value of invading territories at all. On any given turn, the front line will have somewhere between 10-20% of the world economy in it. You can’t conquer the entire front line on one turn, so you’re looking at conquering 5%-10% of the world economy on your turn. But you can’t hold every territory you invade, so now you’re looking at an economic boost of 2%-3% – to be collected on your next turn, not this turn – for launching a major successful attack. It hardly seems worth it; rather than seize territories now, you would often be better off ‘investing’ in rearranging your troops to launch a more decisive blow later in the game, but if your opponent follows the same advice, then ‘later’ may never come.