Assuming black holes exist, it is true that every one of them must spin, since the chance of one forming with exactly 0 angular velocity is 0. Black holes are thought to create a singularity in their centre, which is hidden behind their Schwartzchild radius, or event horizon. It seems to me (without thinking too hard about the maths) that a spinning black hole should produce an infinitely dense ring rather than a single point. A ring spinning around its axis should hold itself open.
What is interesting about this is that a singularity in the form of a ring is an allowable kind of analytic function of 3d space with two or more layers of volumes. By this I mean that you could pass through the ring and into a different version of 3d space without I think violating general relativity. This is a bit like ideas about space-time wormholes but it is different, it doesn't connect two distance areas of space by a shortcut, it is just a particular geometry of space where everywhere is continuous (apart from the ring singularity) but the world through the ring is different from the world if you pass by the ring without going through it. Moreover, there are potentially an unbounded number of layers each time you circle around the ring and through it.
Would make a nice concept for a sci-fi movie.
I wonder if such a physical setup could be simulated... the ring doesn't have to be massive, it could be the size of a door, so long as the ring is infinitely dense (which doesn't mean massive).