By core - I meant things developed by Firaxis as opposed to mods.
To be more clear: simple implementation will cause imbalance between civs, and balancing the feature would require too much effort. So I'd prefer development team to focus on other things and leave this feature to modders.
Balance? On a create map check box option? Who cares?
Not only is this option easy to implement, but it will have absolutely no impact on your game, unless you choose to check the box.
If this were a simple mod, I wouldn't care. It isn't, at least for Civ 4 it wasn't. Would be easy for the devs to implement it as they are making the UI, DLL, etc though.