I don't personally see the problem with having initial terror sites close to the X-COM base. It's already been mentioned on this board that terror sites could only represent those incidents that local government forces can't handle. If we use that assumption, we could assume that local forces are able to contain the initial terror sites reasonably well, and only ask for your help when you're close enough to get there in time. As the threat escalates, the terror sites become harder and harder for local forces to handle, and they're more likely to ask for X-COM support until at last you have sites that only X-COM can handle appearing all over the globe. The need for further reaching forces would be another aspect of increasing difficulty as the game goes on, like the appearance of bigger ships and tougher baddies - and if done right it could improve the sense of desparation.