It is conjectured that a rotating black hole has at its center a ring-shaped singularity.
Thus, at the center of the ring-shaped singularity the gravitational field must be zero (similar to gravitational field at center of dense object), and gravity must be minimized along the rotational axis. At the rotational axis, the gravitional field will be oriented towards the center from both sides, which will cause matter and eventually spacetime to flow towards the center.
Therefore, it seems plausible that the event horizon is a flattened ellipsoid that has upper and lower inward bulges that are rotationally symmetric around the rotational axis.
Also, for a very large black hole, the event horizon might open up around the rotational axis, such that the event horizon becomes toroidal (donut-shaped). Is that possible?