This brings up an on going debate, and many believe the capital rules should be revisited. A good place to start would be with a powers bank can only be taken once per game (when it’s capital is taken). After that you forfeit any IPCs held to the bank. The French are a very good example for this.
Here’s something I have thought about (while YG has been developing Halifax) but haven’t shared. Most capitals start with a major IC (only Rome starts w/minor). What if when you capture an enemy capital you are awarded 2x the IC’s production level in IPCs. If they have less then you take it all, if they have more they keep it (hidden, or relocated to use if they ever get their capital back). Could forfeit any excess to the bank?
In a normal G40 game say G1 you take Paris (capital of France). Its production as a major IC is 10, double that and you would get a max up to 20 IPCs (wouldn’t take damage into account). They start with 19, so that’s what you get (they keep nothing). The second time you take Paris it would have a minor IC on it, so production is 3 units X2= 6 IPCs given to Germany, any thing left over is kept safe for them to use later (or could forfeit to bank). Sack London and the max awarded is 20 IPC to Germany, say the rest was sent to Canada for safe keeping.
This is just me thinking out loud……might work for the new Halifax variant (still under development by Young Grasshopper) or if you wanted to house rule in regional capitals for Calcutta, and joint regional capitals for Sidney, and Ottawa. They all have the new mid level production center that produces 5 units. Double that and if captured the the capturing power would get up to 10 IPCs directly from that powers bank (plus an NO). Some other details would have to be worked out.