It is surprising that Omni haven't made use of the fact that they have all the necessary apps in their stable to create an integrated solution. A mind map is a tree - so is an OF structure. They are just different ways of looking at the same data.
There are lots of useful mind mapping solutions out there (my favourite is also
MindNode). But the killer implementation would be a view of a tree or a sub-tree in OF as a map. And, inversely, a way of importing a mind map from, say, OmniGraffle into OmniFocus.