This would give the game a good healthy dose of realism. To be able to convert (at a fair price) IC from one type-say, CG, into another such as naval IC would be a nice little touch.
Different types of IC would be cool. For example, you'd need naval IC to build ships, and aeroplane IC to build planes. Not sure how deep you'd go into it (Just IC for land, air and sea?), but I think two for each branch (Land, air and sea) would be good. For example, infantry/artillery and armoured IC for the army, small/medium and large plane IC for air, screen and capital IC for naval. Then maybe a general IC for consumer goods and supplies.
I suppose that would make it easier to have things go a bit more historically too (Countries with large naval IC will be building more ships, for example).