Trails traditionally were formed by movements of people as their way of getting around. These days mountain trails are often used for recreation. More people these days live in cities than in rural areas, so it makes sense that trails get formed in cities.
There is a phenonema called 'Desire paths' which are where people cut off the corners of paths because it is more convenient. If you look in fields near to housing estate you will often find that footpaths and bridleways are a lot more worn in towns than out in the countryside. It makes sense that there are trails where there are people which is why I'm surprised that there isn't more out there about off-road style cycle paths in cities. Trails could be created that joined up bits of off-road trail, cycle path, trails through small areas of landscaping on business parks. Most of the infrastructure is there it is just a case of acknowledging that it is a thing and perhaps putting up a few more markers.
There are urban downhill races and road bike races in cities, but what about urban cross country routes in the sense of having mountain bike style singletrack in a city. The benefits of this is that it is cheaper to build, keeps people fit and could make use of otherwise derelict land. Often in cities there are areas that become wild because they are fenced off or not often used by people. These areas give city dwellers the chance to feel wilderness and nature but in the city.
It would be brilliant to see this kind of initiative grow in the future. I think it is a logical intuitive step as more people cycle and more people cycle recreationally to help support off-road bike trails and for authorities to try and understanding this user demographic as much as they are trying to understand and cater for road cyclists.