yes, this should be a manageable task – perhaps a special list plugin:
simplest:
given the current top category, autogenerate a list of it’s child categories
a more powerful version might add control/arguments for:
a) add a ‘subcurrent’ ID to whatever sub category we’re on
b) control whether it displays sub-subcategories/hierarchical levels
c) if b), control how much of the tree ‘above’ the current level is displayed
