To me the thought of a global map is not bad if you can somehow streamline it so it wont be so taxing on the hardware.
I can understand the concern, but I think the the creativity of the mission/s maker can keep think things concentrated within a certain area to keep things interesting. I think a dynamic campaign generator along with some sort of battlefield manager would come in handy to activate/deactivate areas for operations. What do I know really? That was just some off the wall, out of the box thinking. I do think there could be some real possibilities with a "global map" though.