If Al Qaeda somehow eventually ceases to exist as a core functioning terrorist organization, who do you see taking it's place at the forefront of the global jihadist movement? Also whoever takes Al Qaeda's place in this position be the same or even more dangerous then AQ, could it be even be a state supported organization etc? I know again I'm casting a wide net here but in my opinion, such an organization would come out of Pakistan, or more in Central Asia it's self.
There are a lot of opinions on this, and of course there is no consensus because no one really knows (even the Group out there that envisons itself being the next AQ doesn't know if it will succeed). While a new leader may emerge, I don't think a supreme leader is required anymore. AQ kick started the movement globally and provided the ideology. As long as the umbrella ideology still has traction with an audience, then it is probable that the global jihadist struggle will continue (hopefully with less intensity). that there would have to be a leader of the global jihadist movement.

Where will the next leading Islamist terrorist groups come from? It seems probable that that the next major group could emerge from Central Asia, and they may find some degree of State support there as well; however, it is also likely that the next group could emerge in the Horn of Africa, Yemen, Iraq, etc.?

I do agree that future groups could be, and likely will be more dangerous than AQ based on two factors (assuming their ideology supports mass murder):

1. New technologies and knowledge (bioscience for example) available to potential terrorists.

2. Organizations are capable of learning, so in theory a new or emerging group will avoid many of the mistakes that AQ made.

As for State support, that is generally a fickle condition. The U.S. provided support to the Afghan resistance, and it isn't far fetched to assume that Islamic and non-Islamic states may now be providing support to the current Afghan resistance to weaken our position in Central Asia.

One final thought, we should keep in mind that terrorist groups could emerge again from the far left, environmentalis, the far right, etc., not just from jihadists.