There aren't many forums about biscuits - you could try making one of those? What about a knitting forum? Maybe one about fire extinguishers? Sheepdog training? Skydiving? What about owning and caring for emus? Perhaps a forum for millionaires? The Kardashians are popular - have they got a forum?
If none of those appeal to you then simply do what everyone else does and create a forum about a subject you enjoy, are knowledgeable about, and/or have an interest in!