The Fulani jihad states were cavalry-based, so their potential conquests are limited by where horses can go. They could conquer the Sahel and the savanna belt, but the Sahara and coastal West Africa are out unless they diversify their military.
I've suggested in the past that if the Fulani assimilated the peoples of the conquered cities, they might recruit an infantry force from those peoples that would enable them to expand further south. If so, it might not be impossible for them to conquer all West Africa and the Sudan - the logistics aren't much worse than they were for the Mongols or the seventh-century Arabs, and the Fulani would have religion as a powerful unifying force. Of course, this presupposes no interference from outside forces such as the Europeans, so the POD would probably have to involve earlier Fulani jihads.
