That this is the desire of Osama Bin Ladin and his followers sounds very plausible. The caliphate would be a set of leaders who can speak with one voice for Muslims all over the world.
As I understand Muslim theology, once a caliphate is set up, these leaders can declare a jihad to gain new geographical territory for Islam. Perhaps someone else more well-schooled could comment.
If he was acknowledged caliph then yes he could. If he could claim decent from Muhammad (not a requirement to be caliph from the Sunni perspective) then he could also be a legitimate Imam which would go along way in making him unquestionable throughout the Muslim world.
I beieve that islamic divisions of mainline Sunni and Shi'a arms will see this as the radical power grab that it is and start a turf war if such occurs.